BUREAU  OF  AIRCRAFT  PRODUCTION 

•y    |_  INSPECTION  DEPARTMENT 


''  " 


o- 

U3 


FORMATION  FOR 
NSPECTORS  OF 
AIRPLANE  WOOD 


Prepared  at 

FOREST  PRODUCTS  LABORATORY 

FOREST  SERVICE 
U.  S,  DEPARTMENT  OF  AGRICULTURE 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1919     . 


GIFT   OF 


BUREAU  OF  AIRCRAFT  PRODUCTION 

INSPECTION  DEPARTMENT 


INFORMATION  FOR 
INSPECTORS  OF 
AIRPLANE  WOOD 


Prep ared  at 

THE  FOREST  PRODUCTS  LABORATORY 

FOREST  SERVICE 
U.  S.  DEPARTMENT  OF  AGRICULTURE 


to-YOclocfY^      \a\scr 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1919 


\tt(,l 


r 


CONTENTS. 


The  strength  of  timber 5 

Meaning  of  strength 5 

Variability  of  the  strength  of  wood 5 

Wood  nonhomogeneous 5 

Variation  of  strength  with  locality  of  growth 6 

Variation  of  strength  with  position  in  the  tree 6 

Variation  of  strength  with  rate  of  growth 6 

Variation  of  strength  with  amount  of  summerwood 6 

Variation  of  strength  with  specific  gravity 7 

Variation  of  strength  with  moisture  content 11 

Defects  affecting  strength '. : 15 

Diagonal  and  spiral  grain 15 

Knots 17 

Compression  failures 17 

Brashness. 18 

Decay 18 

Internal  or  initial  stresses  in  wood 18 

Wood  fibers  under  stress  in  the  tree 18 

Internal  stresses  produced  during  drying 18 

Initial  stresses  produced  in  assembling 19 

Recommended  methods  for  determining  working  stresses  for  timber  used 

in  airplane  construction 20 

Nature  of  loading 20 

Publications  on  the  mechanical  properties  of  wood 22 

Shrinkage 22 

Amount  of  shrinkage 22 

Shrinkage  of  propeller  stock 23 

Storage  of  stock  before  kiln  drying. 25 

Rules  for  piling  lumber  and  timbers 28 

Kiln-drying  of  wood 30 

Advantages  of  kiln-drying 30 

The  elimination  of  moisture  from  wood 30 

Three  essential  qualities  of  the  dry-kiln. 31 

Defects  due  to  improper  drying 32 

Case-hardening  and  honeycombing 32 

Collapse 33 

Brashness 33 

Testing  of  kilns  for  drying  airplane  stock 34 

Kinds  of  tests 34 

Instruments 34 

Methods 34 

Preliminary  tests 34 

Current  tests 36 

Final  tests 38 

Treatment  of  wood  after  removal  from  the  dry-kiln 40 

Publications  on  kiln-drying  woods „ 41 

3 


QQ31 77 


4  CONTENTS. 

'  .  '        ,  •"  Page. 

Changes  of  moisture  in  wood  with  humidity  of  air 42 

Gluing  of  woods 

Testing  of  animal  glue  for  airplane  propellers 

Kinds  of  tests 

Strength  tests  of  glued  joints - 45 

Viscosity  test - ... - 

Jelly  strength 46 

pdor . -.-- 

Keeping  quality 

Grease  tests. 

Foaming 

Acid  test - .---'-- 

Comparative  results  of  tests  on  glue 47 

Precautions  in  using  glue 

Preparation  of  glue 

Working  temperature 

Clamping  of  glued  joints 

Glue  room  sanitation 

References 

The  structure  and  identification  of  wood 

Heartwood  and  sapwood 

Annual  rings 

Spring  wood  and  summer  wood 

The  structure  of  hardwoods 

The  structure  of  conifers 

Physical  properties  useful  in  identification . . . 

Color....... 52 

Odor  and  taste.-. 

Weight 53 

Grain  and  texture 

Procedure  in  identifying  wood 

Key  for  the  identification  of  wood  useful  for  the  construction  of  airplanes . .        55 

Description  of  woods  in  key 

Publications  on  the  nomenclature  of  woods 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE 

WOOD. 


THE  STRENGTH  OF  TIMBER. 

MEANING   OF    STRENGTH. 

Strength,  in  the  broad  sense  of  the  word,  is  the  summation  of  the 
mechanical  properties  of  a  material  or  its  ability  to  resist  stresses  or 
deformations  of  various  sorts.  While  such  properties  as  hardness, 
stiffness,  and  toughness  are  not  always  thought  of  in  connection  with 
the  term  " strength,"  they  are  unconsciously  included  when,  in  a 
specific  instance,  they  are  important.  This  may  be  illustrated  by 
some  comparisons  of  oak  and  longleaf  pine.  For  floor  beams  or 
posts,  the  pine,  because  of  its  supporting  power  and  stiffness  as  a 
beam,  has  a  slight  advantage  over  the  oak  and  is  considered 
"stronger."  For  handles,  vehicles,  or  implement  parts,  oak,  because 
of  its  greater  toughness,  or  shock-resisting  ability,  is  decidedly  supe- 
rior to  the  pine  and  is  considered  "stronger."  Thus  it  is  seen  that 
the  term  "strength"  may  refer  to  any  one  of  many  properties  or 
combinations  of  properties,  and  is  necessarily  indefinite  in  meaning 
unless  so  modified  as  to  indicate  one  particular  thing.  To  say,  then, 
that  one  species  is  stronger  than  another  is  a  meaningless  statement 
unless  it  is  specified  in  what  particular  respect  it  excels. 

The  term  strength,  in  its  more  restricted  sense,  is  the  ability  to 
resist  stress  of  a  single  kind,  or  the  stresses  developed  in  one  kind  of  a 
constructional  member,  as  strength  in  shear,  strength  in  compression, 
strength  as  a  beam,  strength  as  a  column.  Used  in  this  way,  the 
term  is  specific  and  allows  no  chance  of  confusion. 

VARIABILITY   OF    THE    STRENGTH   OF    WOOD. 

Wood  nonhomogeneous . — Wood  is  exceedingly  variable  as  com- 
pared with  other  structural  materials.  This  variability  is  due  to  a 
number  of  factors,  heretofore  not  well  understood.  For  that  reason 
any  judgment  of  the  strength  of  a  piece  was  felt  to  be  uncertain. 
The  causes  for  variations  in  the  properties  of  wood  can  now  be 
given  and  their  effects  anticipated  within  reasonable  limits.  This 
should  relieve  the  uncertainty.  The  inspector  should  understand 
in  a  general  way  the  factors  causing  variations  and  their  relation 
to  the  strength  of  the  wood. 


6  INFORMATION   FOR   INSPECTORS   OF   AIRPLANE   WOOD. 

Variation  of  strength  with  locality  of  growth. — In  some  cases  the 
locality  of  growth  has  an  influence  on  the  strength  of  the  timber. 
For  example,  tests  show  a  marked  difference  in  strength  between  the 
Rocky  Mountain  and  Coast  types  of  Douglas  fir  in  favor  of  the  Coast 
type. 

This  influence  of  locality  is  usually  overestimated.  Different 
stands  of  the  same  species  grown  in  the  same  section  of  the  country 
may  show  as  great  differences  as  stands  grown  in  widely  separated 
regions,  so  that  as  a  rule  locality  of  growth  can  be  neglected. 

Variation  of  strength  with  position  in  the  tree. — In  some  instances 
specimens  from  different  parts  of  the  same  tree  have  been  found  to 
show  considerable  difference  in  strength.  In  most  cases,  however, 
the  wood  of  the  highest  specific  gravity  has  the  best  mechanical 
properties  regardless  of  its  position  in  the  tree.  Where  this  is  not 
the  case,  the  toughest  or  most  shock-resistant  material  is  found  near 
the  butt.  Above  a  height  of  10  or  12  feet  variations  of  mechanical 
strength  correspond  to  the  variations  of  specific  gravity.  Some 
variations  with  position  in  cross  section  or  distance  from  the  pith  of 
the  tree  have  been  found  which  could  not  be  entirely  accounted  for 
by  differences  in  specific  gravity. 

Variation  of  strength  with  rate  of  growth.— Strength  is  not -definitely 
proportional  to  rate  of  growth,  either  directly  or  inversely. 

Timber  of  any  species  which  has  grown  with  exceptional  slowness 
is  usually  below  the  average  of  the  species  in  strength  values. 

Among  many  of  the  hardwood  species,  material  of  very  rapid 
growth  is  usually  above  the  average  in  strength  properties.  Notable 
exceptions  to  this  are  found,  however,  and  rapid  growth  is  no  assur- 
ance of  excellence  of  material  unless  accompanied  by  relatively  high 
specific  gravity.  This  is  particularly  true  of  ash. 

In  the  coniferous  species,  material  of  very  rapid  growth  is  very 
likely  to  be  quite  brash  and  below  the  average  strength. 

Variation  of  strength  with  amount  of  summer  wood. — In  many 
species  the  proportion  of  summer  wood  is  indicative  of  the  specific 
gravity,  and  different  proportions  of  summer  wood  are  usually  ac- 
companied by  different  specific  gravities  and  strength  values.  How- 
ever, proportion  of  summer  wood  is  not  a  sufficiently  accurate  indi- 
cator of  strength  to  permit  its  use  as  the  sole  criterion  for  the  accept- 
ance or  rejection  of  airplane  material.  After  some  practice  the 
inspector  should  be  able,  through  observation  of  the  proportion  of 
summer  wood,  to  decide  whether  any  particular  piece  is  considerably 
below,  considerably  above,  or  near  the  required  specific  gravity. 
Caution  must  be  observed  in  applying  this  to  ash,  and  perhaps  to 
other  hardwoods,  since  rapid-growth  ash  is  sometimes  very  low  in 
specific  gravity  in  spite  of  a  large  proportion  of  summer  wood.  In 


INFORMATION   FOR   INSPECTORS   OF   AIRPLANE   WOOD.  7 

such  cases  careful  examination  will  show  that  the  summer  wood  is 
less  dense  than  usual. 

Variation  of  strength  with  specific  gravity. — A  piece  of  clear,  sound, 
straight-grained  wood  of  any  species  is  not  necessarily  a  good  stick 
of  timber.  To  determine  the  quality  of  an  individual  stick  by  means 
of  mechanical  tests  is  extremely  difficult,  because  the  variations  in 
strength  of  timber  due  to  variations  in  moisture  content,  tempera- 
ture, speed  of  test,  etc.,  are  so  great.  Furthermore,  a  test  for  one 
strength  property  does  not  always  indicate  what  the  other  properties 
of  the  timber  are.  Without  actual  and  complete  tests,  the  best 
criterion  of  the  strength  properties  of  any  piece  of  timber  is  its 
specific  gravity  or  weight  per  unit  volume,  weight  being  taken 
when  the  wood  is  completely  dry  and  volume  when  the  wood  is  at 
some  definite  condition  of  seasoning  or  moisture  content.  Specifi- 
cation No.  20505A  gives  the  method  to  be  followed  in  obtaining 
specific  gravity  based  on  " oven-dry  volume."  Specific  gravity 
based  on  oven-dry  volume  is  greater  than  that  based  on  the  volume 
at  any  other  moisture  condition  in  proportion  to  the  shrinkage  which 
takes  place  as  the  moisture  is  driven  out  and  the  wood  is  reduced  to 
the  oven-dry  condition. 

Accurate  determinations  made  at  the  Forest  Products  Laboratory 
on  seven  species  of  wood,  including  both  hardwoods  and  conifers, 
showed  a  range  of  only  about  4J  per  cent  in  the  density  of  the  wood 
substance,  or  material  of  which  the  cell  waUs  is  composed.  Since 
the  density  of  wood  substance  is  so  nearly  constant,  it  may  be  said 
that  the  specific  gravity  of  a  given  piece  of  wood  is  a  measure  of  the 
amount  of  wood  substance  contained  in  a  unit  volume  of  it.  Very 
careful  analysis  based  on  the  vast  amount  of  data  available  at  the 
Forest  Products  Laboratory  have  shown  that  wood  of  high  specific 
gravity  has  greater  strength  than  that  of  low  specific  gravity.  Some 
fairly  definite  mathematical  relations  between  specific  gravity  and 
the  various  strength  properties  have  been  worked  out.  Some  of  the 
strength  properties  (strength  in  compression  parallel  to  grain  and 
modulus  of  elasticity)  vary  directly  as  the  first  power  of  the  specific 
gravity;  others,  however,  vary  with  higher  powers  of  the  specific 
gravity,  i.  e.,  the  strength  property  changes  more  rapidly  than  the 
specific  gravity,  a  10  per  cent  increase  of  specific  gravity  resulting 
in  an  increase  in  the  strength  properties  of  15  per  cent  to  even  30 
per  cent. 

The  rate  of  change  in  strength  with  changes  of  specific  gravity 
is  usually  greater  in  individual  specimens  of  a  single  species  than  in 
the  averages  for  a  number  of  species.  This  is  iUustrated  by  a  com- 
parison of  figures  1  and  2.  Figure  1  indicates  that  the  modulus  of 
rupture  varies  as  the  5/4  power  of  the  specific  gravity  when  various 


INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 


species  are  considered,  while  figure  2  indicates  that  the  relation  of 
the  same  property  of  individual  specimens  of  white  ash  is  best 
expressed  by  an  equation  involving  the  3/2  power  of  specific  gravity. 


i« 

H    "• 

,8 

2?il5?v 

: 

-1-.' 

7 

~7_ 

M 

% 

/ 

2j 

* 

^ 

6J^ 

4, 

B 

/ 

610 

n/ 

,'C.lc 

8 

r 



iS 

/ 

;' 

2 

;'-;' 

£ 

a 

IZ6 

/ 

"C 

5 

/ 

0 

fc 

^ 

>|(' 

'" 

/ 

14 

fi1 

•K 

% 

.<• 

IM 

. 

^ 

•^t 

.,, 

ios 

* 

/ 

. 

w 

B 

to 

f 

^/ 

a 

B 

/ 

"l 

^ 

,>... 

J/ 

^!- 

!6j 

/ 

D» 

,. 

J 

^ 

1* 

Mh 

'•1 

: 

" 

. 

-<•» 

y 

91 

,.,. 

5 

•^ 

"'• 

.^ 

« 

B 

V 

132 

^ 

' 

44 

/ 

'V 

/j 

J.  c^ 

J 

iy 

^ 

«0 

0 

X 

;:,H 

"0 

• 

'. 

4f 

.:, 

i 

)f 

;/ 

1 

J"« 

''* 

y 

'"« 

iiM 

M 

ii 

y 

0 

'  f 

1 

L'e 

a,,J 

k- 

1 

? 

-' 

J30 

tfl 

;- 

K 

i 

it  £ 

")'" 

it- 

^0 

• 

n 

^ 

•  • 
/ 

.. 
L, 

* 

tM 

Ijl 

• 

y 

c 

* 

,'; 

/ 

,v 

? 

R, 

* 

6 

'"  * 

V 
• 

*« 

:,'•' 

f. 

• 

"6_ 

•*• 

/ 

ii  U 

4 

^ 

'•V 

ri 

R 

«,• 

•^ 

,1 

/• 

^ 

JC< 

B 

^ 

/ 

. 

• 

' 

/-, 

. 

|/ 

/ 

. 

,g 

L 

I 

? 

••' 

>• 

^ 

••• 

i  i 

^ 

'V 

3_3 

/ 

•ijp 

V' 

f 

'• 

/ 

• 

/ 

/ 

. 

/ 

/ 

/ 

X 

' 

/ 

/ 

G^ 

/ 

x 

REEN)  M.  OF  R.  =  IflSOOVo5" 
JR  DRY]  M.OFR.-26200V 

x; 

x 
x 

^ 

:  ~ 

IA 

\         \         II         1 

0                              OJ                            0.2                           0.3                           <M                           OJ                           0.6                            0.7                           0 

SPECIFIC  GRAVITY 


FIG.  1.— Relation  between  the  modulus  of  rupture  and  specific  gravity  of  various  American  woods. 

Modulus  of  rupture  of  spruce  and  .of  numerous  other  species  has 
been  found  to  vary  as  the  3/2  power  of  the  specific  gravity.  Shock- 
resisting  ability  and  other  important  properties  vary  as  even  higher 
powers  of  specific  gravity.  If  an  important  airplane  part  is  from 


INFORMATION    FOR    INSPECTORS    OF    AIRPLANE    WOOD. 


9 


List  of  species  and  reference  numbers  for  Fig.  1. 

HARDWOODS. 


Species. 

Locality. 

Refer- 
ence 

No. 

Species. 

Locality. 

Refer- 
ence 
No. 

Alder  red 

Washington 

30 

Hickory  —  Continued. 

Ash- 

Pignut 

Pennsv  1  vania 

160 

Biltmore                     .  .  . 

Tennessee  

91 

Pignut  

West  Virginia 

161 

Black 

Michigan 

60 

Shagbark 

Mississippi 

140 

Black          

Wisconsin 

70 

Shagbark  

Ohio  

152 

Blue 

Kentucky 

99 

Shagbark 

Pennsylvania 

143 

Green 

Louisiana 

93 

Shagbark 

West  Virginia 

153 

Green 

Missouri     .   .  . 

100 

Water  

Mississippi     .  . 

141 

Pumpkin 

Missouri 

79 

Hollv,  American 

Tennessee 

87 

White 

Arkansas  

i     106 

Hornbeam  

Tennessee  

149 

White 

Few  York 

'     128 

Laurel,  mountain 

Tennessee 

145 

White         

West  Virginia. 

83 

Locust: 

Aspen 

Wisconsin  

23 

Black 

Tennessee 

158 

Largetooth 

Wisconsin 

20 

Honey 

Indiana 

162 

Basswood 

Pennsylvania 

12 

Madrona  

California 

101 

Basswood 

Wisconsin 

5 

Madrona 

Oregon 

128a 

Beech 

Indiana  

110 

Magnolia  

Louisiana  

66 

Beech  

Pennsylvania  . 

98 

Maple: 

Birch: 

Oregon.  .  . 

Washington.  . 

58 

Paper 

Wisconsin  .. 

73 

Red 

Pennsv  1  vania 

69 

Sweet     

Pennsylvania  . 

129 

Red  

Wisconsin... 

92 

Yellow 

Pennsylvania 

107 

Silver 

Wisconsin 

56 

Yellow  

Wisconsin  

103 

Sugar  

Indiana  

104 

Buckeve,  vellow 

Tennessee  

9 

Sugar 

Pennsylvania 

108 

Buckthorn  cascara 

Oregon 

84a 

Sugar 

Wisconsin 

124 

Butternut      

Tennessee  

27 

Oak: 

Butternut 

Wisconsin 

21 

Bur 

Wisconsin 

125 

Chinquapin,  western  

Oregon  

46b 

California  black 

California 

80 

Cherry: 

Canyon  live  

California  

163 

Black  

Pennsylvania  . 

72 

Chestnut        

Tennessee 

121 

Wild  red 

Tennessee  .  .. 

24 

Cow 

Louisiana 

133 

Chestnut  

Maryland  

46 

Laurel      

Louisiana 

116 

Chestnut 

Tennessee  .. 

40 

Post 

Arkansas 

130 

Cottonwood  black 

Washington 

6 

Post 

Louisiana 

137 

Cucumber  tree           

Tennessee  

59 

Red 

Arkansas 

119 

Dogwood: 

Red... 

Indiana 

118 

Flowering  

Tennessee  . 

151 

Red 

Louisiana 

117 

Western 

Oregon 

125a 

Red 

Tennessee 

97 

Elder,  pale... 

Oregon  

69a 

Highland  Spanish 

Louisiana 

94 

Elm:  ' 

Lowland  Spanish     .  . 

Louisiana 

142 

Cork  

Wiscon  sin, 

126 

Swamp  white 

Indiana 

150 

Marathon 

Tanbark... 

California 

115 

County. 

Water 

Louisiana 

111 

Cork  

Wisconsin, 

White... 

Arkansas    .  .. 

132 

Rusk  County 

White    . 

Indiana 

138 

Slippery  

Indiana  

102 

White  

Louisiana, 

136 

Slippery 

Wisconsin 

74 

White..  

Pennsylvania 

55 

Parish 

White 

Wisconsin 

53 

White 

131 

Greenheart  

165 

Winn  Parish 

Gum: 

Willow 

Louisiana 

109 

Black  

Tennessee.     .  . 

68 

Yellow 

Arkansas 

122 

Blue  (Eucalyptus) 

California 

147 

Yellow 

Wisconsin 

105 

Cotton  

Louisiana  

76 

Osage  orange 

Indiana 

164 

Red  

Missouri 

54 

Poplar  yellow  (tulip  tree) 

Tennessee 

35 

Hackberry 

Indiana 

90 

85 

Hackberry  

Wisconsin 

78 

Sassafras 

Tenne  see 

51 

Haw.  pear  

Wisconsin 

146 

Serviceberry 

Tennessee 

156 

Hickory: 

Silverbell  tree 

TflTlTlfiSSfift 

49 

Bigshellbark  

Mississippi 

135 

Sourwood 

Tennessee 

89 

Big  shellbark 

Ohio 

154 

Sumac  staghorn 

61 

Bitternut  

Ohio     

139 

Sycamore 

Indiana 

63 

Mockernut  

Mississippi 

144 

Sycamore 

65 

Mockernut 

Pennsylvania 

159 

45 

Mockernut 

West  Virginia 

155 

Willow 

Nutmeg  

Mississippi 

112 

Black 

U 

Pignut  

Mississippi  .. 

148 

Western  black 

43a 

Pignut  

Ohio 

157 

Witch  hazel 

114 

10 


INFORMATION  FOE  INSPECTORS  OF  AIRPLANE   WOOD. 


List  of  species  and  reference  numbers  for  Fig.  1 — Continued. 
CONIFERS. 


Species. 

Locality. 

Refer- 
ence 
No. 

Species. 

Locality. 

Refer- 
ence 
No. 

Cedar: 
Incense 

California 

26 

Pine  —  Continued  . 
Lodgepole 

Montana 

40a 

Western  red  

Montana  .     .. 

2 

Jefferson 

Western  red 

Washington 

10 

County 

White 

Wisconsin 

Lodgepole 

34 

Cypress,  bald  

Louisiana 

62 

Longleaf 

Florida 

123 

Douglas  fir 

California 

45a 

Longleaf 

113 

Douglas  fir 

Oregon 

67a 

Lake  Charles 

Douglas  fir 

Washingto  n  , 

46a 

Longleaf 

Louisiana 

96 

Douglas  fir 

Chehalis 
County. 
Washington  , 

75 

Longleaf 

Taneipahoa 
Parish. 

Mississippi 

95 

Lewis  County 

Norway    . 

Wisconsin 

57 

Douglas  fir. 

Washingto  n  , 

67 

Pitch 

Tennessee 

71 

and  Oregon. 

Pond  

Florida 

86 

Douglas  fir  

Wyoming  .. 

84 

Shortleaf 

Arkansas 

77 

Sugar 

California 

22 

Alpine  

Colorado  .  

4 

Table  Mountain  

Tennessee 

82 

Amabilis  .. 

Oregon 

39 

Western  white 

Montana 

42 

Amabilis  

Washington.  . 

18 

Western  yellow  

Arizona  

19 

Balsam  

Wisconsin 

14 

Western  

California 

37 

Grand 

Montana 

36 

Western 

Colorado 

41 

Noble... 

Oregon  

16 

Western.  .  . 

Montana  

32 

White 

California 

17 

White 

Wisconsin 

25 

Hemlock: 

Redwood 

California 

28 

Black  

Montana     .     . 

47 

Albion. 

Eastern 

Tennessee 

52 

Redwood 

California, 

13 

E  astern  

Wisconsin..     . 

15 

Korbel. 

Western  

Washington 

50 

Spruce: 

Larch,  western 

Montana 

84 

Engelmann 

Col  orado,  Grand 

8 

Larch,  western  .  .  . 

Washington 

64 

County. 

Pine: 

E  ngelmann            .  . 

Colorado,  San 

3 

Cuban 

Florida 

127 

Migel  County 

Jack  

Wisconsin...   . 

43 

Red    

New     Hamp- 

44 

Jeffrey 

California 

33 

shire. 

Loblolly  

Florida  

88 

Red... 

Tennessee  

29 

Lodgepole    . 

Colorado     . 

31 

White    .. 

New    Hamp- 

7 

Lodgepole 

Montana  Gal 

35a 

shire 

latin  County. 

White  

Wisconsin  

38 

Lodgepole 

Montana,  Gran- 

41a 

Tamarack 

Wisconsin 

81 

ite  County 

Yew  western 

Washington 

134 

wood  10  per  cent  below  the  specific  gravity  given  in  the  specifica- 
tions, it  will  not  be  just  10  per  cent  but  at  least  14.5  per  cent  inferior 
and  perhaps  more,  depending  on  which  particular  property  is  of  great- 
est importance  in  the  part  in  question.  If  the  specific  gravity  is  20 
per  cent  low,  the  inferiority  will  not  be  less  than  28.4  per  cent.  The 
lighter  pieces  of  wood  are  usually  exceedingly  brash,  especially  when 
dry.  The  importance  of  admitting  no  material  for  airplane  con- 
struction of  lower  specific  gravity  than  given  in  the  specifications  is 
evident. 

The  minimum  strength  values  which  may  be  expected  of  a  par- 
ticular lot  of  lumber  can  be  raised  a  good  deal  by  eliminating  a  rela- 
tively small  portion  of  the  lighter  material.  This  lightweight 
material  can  as  a  rule  be  detected  by  visual  inspection.  In  order  to 
train  the  visual  inspection  and  to  pass  judgment  on  questionable 
individual  pieces,  frequent  specific  gravity  determinations  are 
necessary. 


INFORMATION   FOR  INSPECTORS  OF  AIRPLANE  WOOD. 


11 


A  specific  gravity  determination  is  relatively  simple  to  make, 
and  it  is  probably  a  better  criterion  of  all  the  qualities  of  the  piece 
than  any  single  mechanical  test  which  is  likely  to  be  applied;  also, 
the  specific  gravity  determinations  need  no  adjustment  such  as  would 

MAXIMUM  CRUSHING  STRENGTH  -  IBS.  PER    SQ.     INCH 


0 

! 

\ 

IV 

C 
C 
C 

> 
> 
i 
> 

1 

C 

> 
> 
> 
> 

s 

c 

k 

1 
> 

I 

i 
> 
> 
> 

\ 

r> 

1 

i 
t 
> 

i 

o 

. 

\ 

\ 

> 

D 

V) 

z 

\ 

"O 

m 

"i 

> 
x 

V 

X* 

' 

r> 

33 

* 

-. 

\' 

^ 

-n 

•T 

c 

•*• 

. 

\t 

n 

O 

z 

•H 

\ 

ty 

^ 

n 

n 

\ 

**xP 

r; 

*y 

< 

0 

M 

X 

Z 

ly 

_ 

H 

*s 

£ 

s^ 

V 

\ 

\ 

>. 

Sy 

Ss, 

v> 

' 

V 

m  m 

j 

^ 

s 

52 

• 

1C 

V 

: 

- 

S~       In 

^ 

*; 

i 

0°           *" 

.  . 

•>/j 

I 

^ 

)  ' 

n 

Sog 

- 

-i 

s^ 

3 

j 

•v> 

§ 

'J 

3 

o 

p 

"'^ 

- 

*3         0> 

5 

(' 

^, 

^ 

^ 

'V1 

- 

.  v 

^ 

Af* 

># 

•    ; 

£5 

^^ 

V 

s 

0 

5S 

\ 

\ 

0        . 

s 

-< 

. 

CO 

. 

be  necessary  on  account  of  the  varied  conditions  of  a  mechanical 
test. 

Variation  of  strength  with  moisture  content. — When  a  piece  of  green 
or  wet  wood  is  dried,  no  change  in  mechanical  properties  takes  place 
until  the  fiber-saturation  point  is  reached.  (For  a  definition  of  fiber- 
saturation  point  see  p.  31.)  The  changes  beyond  this  point  for 


12 


INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD. 


small  test  specimens  free  from  defects  and  very  carefully  dried  are 
illustrated  in  figures  3  and  4.  These  figures  show  that  the  moisture 
content  at  the  fiber-saturation  point  differs  for  different  species  and 


1,000 


900 


10 


15         20         25         30         35          40         45         50         55 
MOISTURE-PER    CENT    OF    DRY    WEIGHT. 


60 


70 


FIG.  3.— Relation  between  the  stiffness  (modulus  of  elasticity)  in  bending  and  moisture  content,  for  three 

species. 

that,  apparently,  the  increase  of  strength  does  not  in  all  cases  begin 
at  the  fiber-saturation  point.  It  will  be  noted  that  the  influence 
of  moisture  is  smaller  in  tests  of  shearing  strength  and  compression 


INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD. 


13 


perpendicular  to  the  grain  than  in  bending  and  compression  parallel 
to  the  grain.  Furthermore,  there  is  no  definite  break  at  or  near  the 
fiber-saturation  point  in  the  moisture-strength  curves  for  shear  and 


1,000 


50 


5  10          15         20         25         30         35         40         45 

MOISTURE- PER    CENT    OF   DRY    WEIGHT. 

FIG.  4.— Comparison  of  the  relations  between  strength  and  moisture  content  for  red  spruce  in  various 
kinds  of  tests.    (The  lowest  curve  is  for  compression  at  right  angles  to  grain.) 

compression  perpendicular  to  the  grain.  In  the  case  of  shear  this 
failure  to  show  large  increases  in  strength  is  probably  due  to  checks 
which  form  as  the  material  dries. 


14  INFORMATION   FOB  INSPECTORS  OF   AIRPLANE   WOOD. 

The  moisture  content  at  the  fiber-saturation  point  varies  not  only 
with  the  species  but  with  different  specimens  of  the  same  species. 
The  percentage  change  of  strength  which  results  from  a  given  change 
of  moisture  also  varies  with  the  species  and  with  individual  sepci- 
mens  of  the  species. 

The  form  of  the  curves  shown  in  figures  3  and  4  applies  only  to 
small  clear  pieces  very  carefully  dried  and  having  a  practically  uni- 
form moisture  content  throughout.  If  the  moisture  be  unequally 
distributed  in  the  specimen,  as  in  the  case  of  large  timbers  rapidly 
dried  or  of  "case-hardened"  pieces,  the  outer  shell  may  be  drier  than 
the  fiber-saturation  point  while  the  inside  still  contains  free  water. 
The  resulting  moisture-strength  curve  will  be  higher  than  the  correct 
curve  and  be  so  rounded  off  from  the  driest  to  the  wettest  condition 


<  P.obo 

T  1Z.OOO 


r 

<~  10,000 


9.000 

fl.000 

7.000 

.  6,000 


Q 

5^.000 


FIG  5  —Effect  of  case-hardening  upon  the  form  of  the  moisture-strength  curve  in  bending  tests.    The 
'  upper  curve  is  from  casehardened  specimens,  the  lower  curve  from  uniformly  dried  specimens. 

as  to  obscure  entirely  the  fiber-saturation  point.     (See  fig.  5.)     A 
fuller  discussion  of  case  hardening  is  given  on  page  32. 

The  increase  in  strength  which  takes  place  in  drying  wood  depends 
upon  the  specimen  and  upon  the  care  with  which  the  drying  process 
is  carried  out.  Furthermore,  while  the  strength  of  the  fibers  is  no 
doubt  greatly  increased  by  any  reasonable  drying  process,  the  in- 
crease of  the  strength  of  a  piece  of  timber  taken  as  a  whole  may  be 
very  much  less.  Knots  are  more  or  less  loosened,  checking  takes 
place,  and  shakes  are  further  developed.  In  large  bridge  and  build- 
ing timbers  these  effects  are  so  great  that  it  is  not  considered  safe  to 
figure  on  such  timbers  having  greater  strength  when  dry  than  when 
green.  When  the  pieces  are  small  and  practically  free  from  defects, 
as  in  airplane  construction,  proper  drying  with  careful  control  of 
temperature  and  humidity  increases  the  strength  of  material  very 


INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD.  15 

greatly.  In  whatever  way  wood  is  dried,  upon  its  being  resoaked 
and  brought  back  to  the  original  green  or  wet  condition  it  is  found 
to  be  weaker  than  it  was  originally.  So  when  it  is  said  that  wood 
has  been  injured  in  the  drying  process,  it  must  be  taken  to  mean  that 
it  is  weaker  than  it  should  have  been  after  drying  and  while  still  in 
a  dried  condition. 

When  a  stick  of  timber  dries  out  below  the  fiber-saturation  point 
(that  is,  when  it  has  lost  all  its  free  moisture  and  the  moisture  begins 
to  leave  the  cell  walls)  the  timber  begins  to  shrink  and  change  in  its 
mechanical  properties.  Also  numerous  stresses  are  set  up  within  the 
timber.  Under  severe  or  improper  drying  conditions  the  stresses 
may  be  great  enough  to  practically  ruin  the  material  for  purposes 
where  strength  is  important.  Improper  drying  conditions,  however, 
do  not  of  necessity  mean  fast  drying  conditions.  When  properly 
dried,  the  timber  gains  in  its  fiber  stress  at  elastic  limit,  its  modulus 
of  rupture,  maximum  crushing  strength,  etc.  It  bends  farther  at  the 
elastic  limit  when  dry  than  when  green,  but  does  not  bend  so  far  at 
the  maximum  load.  After  having  been  sent  to  the  maximum  load 
dry  timber  breaks  more  suddenly  than  green  timber  of  the  same 
species;  that  is,  dry  timber  is  more  brash  than  green,  although  it 
withstands  greater  stresses  and  is  stiffer. 

DEFECTS    AFFECTING   STRENGTH. 

Diagonal  and  spiral-grain. — Diagonal  grain  is  produced  when  the 
saw  cut  is  not  made  parallel  to  the  direction  of  the  fibers.  It  can 
usually  be  avoided  by  careful  sawing  unless  it  is  caused  by  crooks  in 
the  log.  Spiral-grain,  on  the  other  hand,  results  from  a  spiral  ar- 
rangement of  the  wood  fibers  in  the  tree.  If  a  log  is  spiral-grained 
it  is  impossible  to  secure  straight-grained  material,  except  in  small 
pieces,  from  the  spiral-grained  part.  The  center  part  of  a  log  may 
be  straight-grained  and  the  outer  part  spiral-grained  or  vice  versa. 

Such  tests  as  have  been  made  on  material  affected  by  diagonal  and 
spiral-grain  indicate  that  weakening  begins  at  a  slope  of  about  1  in 
20  and  increases  quite  rapidly  as  the  slope  becomes  steeper.  Ex- 
perience in  testing  such  material  also  shows  that  spiral-grain  is  more 
dangerous  than  diagonal  grain. 

When  a  beam  in  a  horizontal  position,  as  shown  in  figure  6,  is  sub- 
jected to  a  vertical  load,  grain  running  across  the  vertical  faces  at  a 
given  slope  is  more  dangerous  than  grain  at  the  same  slope  across 
the  horizontal  faces.  It  is  preferable  to  give  the  annual  layers  or 
rings  of  growth  the  direction  relative  to  load  shown  in  figure  6  in 
material  affected  by  spiral-grain,  in  order  to  minimize  the  weakening 
effect  of  the  spiral  grain. 
84727—19 2 


16 


INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 


When  the  annual  rings  run  diagonally  across  the  end  of  a  piece  the 
true  slope  of  diagonal  grain  can  be  obtained  as  shown  by  figure  7a. 
The  direction  of  spiral-grain  is  not  given  correctly  by  resin  ducts  or 
by  spreading  of  ink  unless  these  tests  be  applied  to  a  truly  tangential 
(flat  sawn)  face.  In  figure  7,  for  instance,  resin  ducts  or  spreading 


of  ink  would  be  practically  parallel  to  the  edges  whether  the  material 
was  spiral-grained  or  not.  In  such  cases  spiral-grain  can  be  detected 
only  by  splitting  on  a  radial  line  (fig.  7b),  or  by  raising  small  splinters 
and  observing  if  they  have  a  tendency  to  tear  deeper  and  deeper. 


INFORMATION    FOE    INSPECTORS   OF   AIRPLANE   WOOD. 


17 


Knots. — The  effect  of  knots  depends  upon  their  location  with 
respect  to  the  stresses  to  which  the  piece  will  be  subjected,  as  well  as 
upon  their  size  and  character.  None  but  sound  knots,  firmly 
attached,  should  be  permitted.  Obviously,  knots  of  any  considerable 
size  can  not  be  allowed  in  any  airplane  parts  because  the  parts  them- 
selves are  comparatively  small  in  cross-section.  Since  the  weakening 
effect  of  knots  results  from  their  disturbance  of  the  normal  arrange- 
ment of  fibers,  their  seriousness  can  best  be  decided  from  a  considera- 


5 1  ope  of  diagona/ 
gra/n. 


(b) 


=  5/ope   of  spiral 
grain. 


FIG.  7.— The  measurement  of  the  slope  of  diagonal  and  spiral  grain. 

tion  of  the  grain.      For  description  of  defects  in  grain  consult  Inspec- 
tion Manual,  section  QT-lOa. 

Compression  failures . — All  material  containing  compression  failures 
should  be  carefully  eliminated  where  shock-resisting  ability  is  of 
importance.  Such  failures,  fortunately,  are  not  of  very  common 
occurrence.  They  may  be  due  to  injury  by  storm  in  the  standing 
tree,  to  carelessness  in  felling  trees  across  logs,  or  to  unloading  from 
a  car  upon  a  single  skid,  or  they  may  result  from  injury  during  manu- 
facture. Failures  in  hickory  spokes  attributed  to  brittleness  have 


18  INFORMATION   FOE   INSPECTOKS   OF   AIRPLANE   WOOD. 

been  found  in  some  cases  to  have  been  due  to  compression  failures 
which  occurred  during  driving.  Brittleness  sometimes  reported  in 
mahogany  veneer  may  be  due  to  injury  of  the  material  while  it  is 
being  brought  to  the  mill. 

While  some  compression  failures  are  so  pronounced  as  to  be  un- 
mistakable, others  are  difficult  to  detect.  They  appear  as  wrinkles 
across  the  face  of  the  piece.  (See  Fig.  8.)  Compression  failures,  not 
readily  apparent  to  the  eye,  may  seriously  reduce  the  bending  strength 
of  wood  and  its  shock-resisting  ability,  complete  failure  occurring 
suddenly  along  the  plane  of  injury. 

Brashness. — The  term  "brash,"  frequently  used  interchangeably 
with  the  term  "brittle,"  when  used  to  describe  wood  or  failures  in 
wood,  indicates  a  lack  of  toughness.  Brash  wood,  when  tested  in 
bending,  breaks  with  a  short,  sharp  fracture  instead  of  developing  a 
splintering  failure,  and  absorbs  a  comparatively  small  amount  of 
work  between  the  elastic  limit  and  final  failure.  In  impact  tests 
brash  wood  fails  completely  under  a  comparatively  small  hammer 
drop. 

Decay. — The  first  effect  of  decay  is  to  reduce  the  shock-resisting 
ability  of  the  wood.  This  may  take  place  to  a  serious  extent  before 
the  decay  has  sufficiently  developed  to  affect  the  strength  under 
static  load  or  to  become  evident  on  visual  inspection.  Unfortunately, 
there  is  no  method  of  detecting  slight  decay  in  wood  except  with  a 
compound  microscope.  All  stains  and  discolorations  should  be 
regarded  with  suspicion  and  carefully  examined.  It  must  be  remem- 
bered that  decay  often  spreads  beyond  the  discoloration  caused  by 
t  and  pieces  adjacent  to  discolored  areas  may  already  be  infected. 
On  the  other  hand,  not  all  stains  and  discolorations  are  caused  by 
decay  of  the  wood.  The  blue  sapstain  of  some  hardwoods  and  of 
many  coniferous  woods,  including  spruce,  and  the  brown  stain  of 
sugar  pine  are  not  caused  by  decay-producing  organisms  and  do  not 
weaken  the  wood. 

INTERNAL   OR    INITIAL    STRESSES    IN    WOOD. 

Wood  fibers  under  stress  in  the  tree. — Wood  products  are  quite 
similar  to  metal  castings  as  regards  internal  stresses.  It  is  probable 
that  wood  fibers  are  continually  under  stress  of  some  kind.  The  fact 
that  freshly  cut  logs  of  some  species  split  through  the  center  (this 
frequently  happens  as  the  result  of  heavy  shocks  or  jars  and  without 
the  use  of  a  wedge)  is  evidence  of  some  tensile  stresses  in  the  outer 
portion  of  the  tree  and  compression  in  the  inner  portion..  These 
stresses  are  independent  of  the  stresses  due  to  the  weight  of  the  tree 
and  pressure  against  it. 

Internal  stresses  produced  during  drying. — The  natural  stresses  may 
be  partially  or  wholly  relieved  by  sawing  the  tree  into  lumber,  but 


FIG.  8.— Compression  failure  in  a  piece  of  yellow  pine  flooring. 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE   WOOD.  19 

other  stresses  are  likely  to  be  introduced  by  subsequent  seasoning. 
Checking,  honeycombing,  warping,  twisting,  etc.,  are  manifestations 
of  the  internal  stresses  which  are  produced  in  the  drying  of  wood  or 
whenever  any  change  of  moisture  content  takes  place.  Presumably 
such  stresses  are  due  to  unequal  distribution  of  moisture  and  con- 
sequent unequal  shrinkage  combined  with  more  or  less  inherent  lack 
of  homogeneity. 

Air  drying  for  a  number  of  years,  which  is  practiced  in  some 
woodworking  industries,  has  for  its  object  the  equalization  of  mois- 
tures and  the  relief  of  the  stresses  induced  in  the  early  part  of  the 
drying.  Careful  and  correct  kiln  drying  followed  by  a  period  of 
seasoning  under  proper  and  controlled  atmospheric  conditions  should 
produce  results  at  least  equal  and  probably  superior  to  those  obtained 
by  long  periods  of  air  drying. 

Relieving  these  internal  stresses  is  important  because  they  amount 
to  an  actual  weakening  of  the  material.  If  the  fibers  of  a  piece  of 
wood  are  under  stress  when  the  piece  is  free,  they  are  just  that  much 
less  capable  of  resisting  stresses  of  the  same  kind  produced  by  exterior 
forces  or  loads  applied  to  the  piece. 

Initial  stresses  produced  in  assembling. — When  a  member  of  any 
structure  is  stressed  in  assembling  the  structure  and  before  any  load 
is  placed  on  it,  it  is  said  to  be  under  initial  stress.  If  the  initial  stress 
is  of  the  same  character  as  the  stress  for  which  the  member  is  designed, 
it  constitutes  a  weakening;  for  when  the  structure  is  loaded,  the  safe 
working  stress  of  the  member  will  be  reached  just  that  much  sooner. 
If  this  initial  stress  is  opposite  in  character  to  that  for  which  the 
member  is  designed,  it  amounts  to  a  strengthening  of  the  member 5 
for  when  the  structure  is  loaded  the  initial  stress  must  be  overcome 
before  the  member  takes  any  of  the  stress  for  which  it  is  designed. 

Many  of  the  curved  parts  of  an  airplane  frame  could  be  simply 
sprung  to  place  on  assembly.  Were  this  done,  they  would  be  sub- 
jected to  initial  stress  and  usually  of  the  same  sign  to  which  the 
member  would  later  be  subjected.  In  order  to  avoid  initial  stress, 
such  parts  are  steam  bent  before  assembly.  It  is  desirable,  of  course, 
that  this  bending  be  so  done  as  not  to  injure  the  material  and  to  leave 
little  tendency  to  spring  back  from  the  curves  to  which  it  is  bent. 
In  order  that  the  material  may  be  made  sufficiently  plastic  to  ac- 
complish this  result  it  is  essential  that  the  steaming  and  bending  be 
carried  out  while  the  wood  is  at  a  relatively  high  moisture  content. 
If  it  is  attempted  on  kiln-dry  or  thoroughly  air-dry  material,  there  is 
the  tendency  to  spring  back  after  the  clamps  are  removed.  Bending 
of  such  stock  cannot  be  compared  to  a  considerable  part  of  the 
bending  done  in  other  woodworking  industries,  where  the  strength 
of  the  wood  is  very  greatly  damaged  by  the  bending  process  but 
without  destroying  its  usefulness  for  the  purpose  for  which  it  is 


20  INFORMATION   FOB  INSPECTORS   OF   AIRPLANE   WOOD. 

intended.  Some  of  the  unexpected  failures  of  bent  parts  in  airplanes 
have  doubtless  been  due  to  the  initial  stresses  set  up  in  the  member 
during  the  bending. 

RECOMMENDED   METHODS   FOR   DETERMINING   WORKING    STRESSES    FOR 
TIMBER   USED   IN   AIRPLANE    CONSTRUCTION. 

Table  1  gives  strength  values  at  15  per  cent  moisture  (which  is 
probably  close  to  the  maximum  moisture  content  of  wood  in  a  humid 
atmosphere)  for  use  in  airplane  design,  as  well  as  the  minimum  spe- 
cific gravity  and  average  density  which  should  be  admitted.  It  is  sug- 
gested that  the  working  stresses  for  design  be  obtained  by  applying 
factors  to  the  values  for  static  load  conditions  rs  given  in  this  table. 

The  factors  to  be  applied,  and  consequently  the  exact  stress  to  be 
used  in  design,  of  course,  will  depend  largely  on  the  conditions  to 
which  it  is  assumed  the  machine  will  be  subjected  in  flight.  If  they 
are  the  most  severe  which  the  machine  is  ever  expected  to  sustain 
while  in  flight,  the  working  stresses  can  be  relatively  high.  If,  on 
the  other  hand,  the  assumed  conditions  are  only  moderately  severe, 
the  stresses  must  be  made  lower  in  order  to  take  care  of  exceptional 
conditions  which  may  occur.  It  must  also  be  remembered  that 
working  stresses  cannot  be  safely  based  on  average  strength  figures, 
but  must  be  lowered  to  a  value  which  will  be  safe  for  the  weakest 
piece  likely  to  be  accepted. 

Nature  of  loading. — The  time  of  duration  of  a  stress  on  a  timber  is  a 
very  great  factor  in  the  size  of  the  stress  which  will  cause  failure.  A 
continuously  applied  load,  greater  in  amount  than  the  fiber  stress  at 
elastic  limit  as  obtained  by  the  ordinary  static  bending  test,  will 
ultimately  cause  failure. 

The  fiber  stress  at  elastic  limit  in  static  bending  for  the  dry  material 
is  usually  somewhat  more  than  nine-sixteenths  of  the  modulus  of 
rupture,  and  in  compression  parallel  to  the  grain  the  elastic  limit  is 
usually  more  than  two-thircls  of  the  maximum  crushing  strength. 
Timber  loaded  slightly  below  the  elastic  limit  will  gradually  give  to 
loads  and  ultimately  assume  greater  deflections  than  those  computed 
by  using  the  ordinary  modulus  of  elasticity  figures.  In  impact  tests 
where  a  weight  is  dropped  on  the  stick  and  the  stress  lasts  for  only 
a  small  fraction  of  a  second,  the  stick  is  found  to  bend  practically 
twice  as  far  to  the  elastic  limit  as  in  static  tests  where  the  elastic 
limit  is  reached  in  about  two  minutes.  The  elastic  stress  developed 
in  the  stick  under  the  blow  is  greater  than  the  maximum  stress 
obtained  in  the  static  test. 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD. 


21 


C^t^-Oi        CO 


»O  t—  tO  iO        C<1^ 


>00000i 

>  o  o  o  o  o  < 


OiOO       •f 


IN        Ot~ 


2.gj.gsfl   g^ 

.  d  p  ^  ^  s-,  -u>  »      to.-. 


>OO        OOO  OOOOc 


C'^3 


•ss 


.-<  M<  CO  CO  O  W  CO  O         0.-H 


r-«N         00.-IT-I        rj< 


en    ,  vi 


lOOOOOOOO       OOOO       OOO  OOOOO        O( 


I 


cT     co'cTo'c^r    c*u>i-r        t^cTco'tfror    t^^t^ 


»H  o       ^  ^. 

"  J2  O          Ml         OT  "H 


t^  10  •*>  t>-  oo  t>  •«»<  o  o     oot^t^oo 


tf>  o  co  oo  o  t-  05  oo  c<i      co»ooooo     COI-H 


4_J    0) 


rt<      TJIC<I 


c^ic^es)oo     <NW 


N«iS 

°Ntl§J 


«O  00  CO  O  ^-i  00  OS  O  00       COOOO       >O  00  C<«  O3C^r-iO5t~        COO«OCO 

>0  TF  CO  CO  CO  rj.  CO  (O  TJ<        t^0^<o        CDCOiO  CO  -<f  CC  (N  •*        CO  ^  CO  •* 


I! 


<N  ceo  ot^  co  co  CD  in     i-i  •*  o  co     cqeqco          «OI^-<JIIM<N      onco3>-i     1-11^ 

CO  "O  -f  CO  CO  iO  rf  CO  »O       00  "5  iO  CO       t~-  TI<  10  CO  •*  tt  CO  1C       OO  TJI  CO  m       •&•*)< 

o 


11 


II 


iil 


f  SEi^s  'xlarfsl 

Illfl  iflllP 


22  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

PUBLICATIONS    ON   THE    MECHANICAL    PROPERTIES    OF    WOOD. 

1.  Government  publications: 

Mechanical  Properties  of  Wood  Grown  in  the  United  States.  Department  of  Agri- 
culture Bulletin  556.  1917.  10  cents. 

Strength  of  Wood  as  Influenced  by  Moisture.  Forest  Service  Circular  108.  1907. 
5  cents. 

Timber:  An  Elementary  Discussion  of  the  Characteristics  and  Properties  of  Wood. 
Forest  Service  Bulletin  10.  1895.  10  cents. 

The  Commercial  Hickories.     Forest  Service  Bulletin  80.     1910.     15  cents. 

Tests  of  Structural  Timbers.     Forest  Service  Bulletin  108.     1912.    20  cents. 

Properties  and  Uses  of  Douglas  Fir.     Forest  Service  Bulletin  88.     1911.     15  cents. 

NOTE. — The  above  publications  may  be  obtained  at  the  prices  indicated  from  the  Superintendent  of 
Documents,  Government  Printing  Office,  Washington,  D.  C. 

2.  Papers  prepared  by  the  Forest  Products  Laboratory  and  published  in  various 
journals. 

Variation  in  Weight  and  Strength  of  Timber.  J.  A.  Newlin,  in  American  Lumber- 
man, January  22,  1916. 

Effects  of  Different  Methods  of  Drying  on  Strength  of  Wood.  H.  D.  Tiemann,  in 
Lumber  World  Review,  April  24,  1915. 

A  Few  Deductions  from  Strength  Tests  of  American  Woods.  J.  A.  Newlin,  in  Ameri- 
can Lumberman,  January  16,  1915. 

Factors  Affecting  Structural  Timbers.  H.  S.  Betts,  in  Engineering  Record,  August  29, 
1914. 

SHRINKAGE. 

AMOUNT    OF    SHRINKAGE. 

Ordinarily,  when  a  piece  of  green  lumber  is  dried  no  change  in 
dimensions  takes  place  until  the  fiber  saturation  point  is  reached. 
The  wood- then  begins  to  shrink  in  cross-sectional  area  until  no  further 
moisture  can  be  extracted  from  the  cell  walls.  It  also  shrinks 
longitudinally,  but  in  most  cases  the  amount  of  longitudinal  shrink- 
age is  so  small  as  to  be  negligible. 

The  shrinkage  in  cross-sectional  area  in  drying  from  the  green  to 
the  oven-dried  condition,  varies  with  different  woods,  ranging  from 
as  much  as  22  per  cent  (based  on  the  original  area  before  drying 
begins)  to  as  little  as  6  per  eent.  When  dry  wood  absorbs  moisture 
it  continues  to  swell  until  the  fiber  saturation  point  is  reached. 
Figures  9,  10,  and  11  illustrate  the  progress  of  shrinkage  and  swelling 
between  zero  moisture  content  and  the  fiber  saturation  point. 

The  shrinkage  of  wood,  like  its  strength,  is  very  closely  related  to 
its  specific  gravity.  This  is  illustrated  by  figure  12.  On  this  curve 
"Per  cent  shrinkage  in  volume"  is  the  total  shrinkage  from  fiber 
saturation  to  dryness.  It  will  be  noted  that  shrinkage,  in  general, 
increases  with  specific  gravity.  This  relation  in  individual  speci- 
mens of  a  single  species  (white  ash)  is  shown  in  figure  13. 

Radial  shrinkage,  or  the  shrinkage  in  width  of  quarter  sawn 
boards,  averages  about  three-fifths  as  great  as  tangential  shrinkage, 
or  the  shrinkage  in  width  of  flat  sawn  boards. 


INFORMATION    FOR   INSPECTORS   OF   AIRPLANE   WOOD. 


23 


SHRINKAGE  OF  PROPELLER  STOCK. 

Shrinkage  and  the  various  phenomena  associated  with  it  are  of 
immense  importance  in  connection  with  propeller  manufacture.  The 
following  precautions  in  manufacturing  propellers  will  assist  in  reduc- 
ing to  a  minimum  trouble  from  failure  of  glued  joints,  splitting  of 
laminations,  change  of  pitch,  and  imperfect  alignment. 

1.  All  material  should  be  quarter-sawed  if  possible. 

2.  Quarter-  and  flat-sawed  laminae  should  not  be  used  in  the  same 
propeller. 


/^ 
/<? 
S 

**  5 
»  -^ 
1-2 
^  O 


< 


<Q- 


:/^ 


5     6 


/*    J6    JB 


28  'X   3Z 


36    38   4O 


FIG.  9.—  Relation  between  swelling  and  moisture.  Each  point  is  the  average  of  from  five  to  eleven  speci- 
mens. Black  dots  indicate  specimens  that  were  kiln-dried  and  then  allowed  to  reabsorb  moisture.  The 
fiber-saturation  point  is  at  c. 

3.  All  laminae  should  be  brought  to  the  same  moisture  content 
before  gluing  up. 

4.  All  laminae  in  the  same  propeller  should  have  approximately 
the  same  specific  gravity. 

5.  All  laminae  in  the  same  propeller  should  be  of  the  same  species. 
Dry  wood  when  exposed  to  very  humid  air  absorbs  moisture  and 

swells.  Wood  dried  in  a  normally  dry  atmosphere  till  its  moisture 
content  becomes  practically  constant,  loses  moisture  and  shrinks 
when  exposed  to  extremely  dry  conditions.  Two  pieces  of  wood  when 
exposed  continuously  to  the  same  environment  will  eventually  come 


24 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD. 


to  practically  the  same  moisture  content,  irrespective  of  their  rela- 
tive moisture  contents  when  first  exposed  to  this  environment. 

Individual  pieces  of  wood,  even  those  of  the  same  species,  vary 
greatly  in  their  rate  of  drying.  Quarter-sawed  pieces  have  a  different 
drying  rate  from  plain-sawed  pieces.  Dense  pieces  dry  more  slowly 
than  those  which  are  less  dense. 

Suppose  that  a  flat-sawed  board  is  glued  between  two  quarter- 
sawed  boards,  all  three  having  the  same  moisture  content,  say,  15 
per  cent,  when  glued  up;  or,  suppose  that  under  similar  conditions 
a  very  dense  piece  is  glued  between  two  pieces  which  are  less  dense; 
or,  suppose  that  a  board  containing  15  per  cent  moisture  is  glued 
between  two  others,  each  containing  10  per  cent  but  all  three  being 


DRYING  CURVE 


|,07 

5 


REiBSORPTlON  CURVE 


5          10          IS          20         85         30          35         4O          45         CO 

MOISTURE  PERCENT 

FIG.  10.— Relation  between  the  moisture  content  and  the  cross  section  of  small,  clear  pieces  of  western 

hemlock. 

of  the  same  density  and  cut  in  the  same  manner.  Then  suppose  the 
finished  product  to  be  dried  to,  say,  8  per  cent  moisture.  Every 
piece  will  shrink,  but  in  each  instance  the  center  piece  will  tend  to 
shrink  more  than  the  outside  ones.  The  glued  joint  will  be  under  a 
shearing  stress,  since  the  center  piece  has  a  tendency  to  move  with 
respect  to  those  on  the  outside.  Under  this  condition  the  glued  joint 
may  give  way  entirely,  it  may  partially  hold,  or  it  may  hold  per- 
fectly. In  either  of  the  latter  cases  the  center  piece  will  be  under 
stress  in*  tension  across  the  grain,  and  consequently,  will  have  a 
tendency  to  split.  This  tendency  may  become  localized  and  result 
in  visible  splitting,  or  it  may  remain  distributed  and  cause  a  lessening 
of  the  cohesion  between  the  wood  fibers,  but  without  visible  effect. 

If  a  combination  of  these  three  cases  occurs,  it  may  be  much  more 
serious  in  its  effect  than  any  one  alone.     For  instance,  suppose  that 


INFORMATION   FOE   INSPECTORS  OF  AIRPLANE   WOOD. 


25 


in  a  propeller  alternate  laminations  are  of  flat-sawed,  dense  boards, 
glued  at  a  relatively  high  moisture  content,  while  the  others  are 
quarter-sawed,  less  dense,  and  at  a  much  lower  moisture  content  when 
glued;  the  tendency  of  the  flat-sawed  laminations  to  shrink  will  be 
very  much  greater  than  that  of  the  others,  with  the  result  that 
internal  stresses  of  considerable  magnitude  will  be  set  up. 

It  is  not  difficult  to  see  how  these  internal  stresses  may  combine 
with  the  stresses  from  external  causes  and  with  the  continual  vibra- 
tion to  produce  failure  under  external  loads  which  are  considerably 
smaller  than  the  propeller  would  safely  resist  if  manufactured  with 
proper  care. 


DRYING    CURVE 


ABSORPTION  FOINT 


MOISTURE   PERCENT 

FIG.  11.— Relation  between  the  moisture  content  and  the  cross  section  of  small,  clear  specimens  of  western 

larch.  - 

STORAGE  OF  STOCK  BEFORE  KILN-DRYING. 

It  may  be  necessary  to  have  timber  or  lumber  in  storage  several 
weeks  or  months  before  it  is  kiln-dried.  Such  stock  is  usually  either 
green  or  only  partly  air  seasoned  and  is  subject  to  various  forms  of 
deterioration,  such  as  staining,  decay,  severe  checking,  and  especially 
in  hardwood,  insect  attack.  During  warm  humid  weather  staining 
may  take  place  in  a  few  days  and  decay  may  weaken  the  wood  in  a 
few  months. 

Proper  piling  of  such  stock  will  tend  to  reduce  the  deterioration  to 
a  minimum.  All  lumber  or  timber  which  is  to  be  stored  any  length  of 
time  should  be  piled  on  solid  foundations,  with  stickers  between  each 


26 


INFORMATION   FOE  INSPECTORS   OF   AIRPLANE   WOOD. 


two  courses,  and  should  have  some  protection  from  the  sun  and  rain. 
Whenever  possible,  the  stock  should  be  piled  in  a  shed  with  open 
sides.  If  this  is  not  practicable,  each  pile  should  be  covered  so  as 


-SHRINKAGE  -  28G 


'•^ 


0.3  0.4 

SPECIFIC     GRAVITY 


FIG.  12. — Relation  between  shrinkage  in  volume  and  specific  gravity  of  various  American  woods. 

to  keep  out  rain  and  snow.  Green  hardwoods,  especially  oak,  fre- 
quently check  severely  at  the  ends.  This  can  be  avoided  to  a  large 
extent  by  coating  the  ends  with  linseed-oil  paint. 


INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

List  of  species  and  reference  numbers  for  Fig.  13. 
HARDWOODS. 


27 


Species. 

Locality. 

Refer- 
ence 
No. 

Species. 

Locality. 

Refer- 
ence 
No. 

Alder,  red  
Ash- 

Washington... 

30 

Hickory—  Continued 
Pignut  

Pennsvlvania 

160 

Biltmore  

Tennessee  

91 

Pignut  

West  Virginia. 

161 

Bla?k... 

Michigan 

60 

Shagbark 

Mississippi 

140 

B  aek 

Wisconsin 

70 

Shagbark 

152 

Blue  

Kentucky 

99 

Shagbark  

Pennsylvania  . 

143 

Green 

Louisiana 

93 

Shagbark 

West  Virginia 

153 

Green  

Missouri   

100 

Water  

Mississippi  

141 

Pumpkin  . 

Missouri  . 

79 

Ho'ly,  American 

Tennessee 

87 

White 

Arkansas 

106 

Hornbeam 

Tennessee 

149 

White... 

New  York 

128 

Laure',  mountain 

Tennsssee  

145 

White 

West  Virginia 

83 

Locust" 

Aspen 

Wisconsin 

23 

Bla"k 

Tenn3SS3e 

158 

Largetooth 

Wisconsin 

20 

Honey 

Indiana 

162 

Basswood 

Pennsylvania 

12 

Madrona 

Ca  ifornia 

101 

Basswood  .  .  . 

Wisconsin 

5 

Madrona  ... 

Oregon  

128a 

Beech  

Indiana 

110 

Magnolia 

Louisiana 

66 

Beech 

98 

Map'e* 

Birch: 

Oregon 

Washington 

58 

Paper 

Wisconsin 

73 

Red 

Pennsylvania 

69 

Sweet... 

Pennsylvania 

129 

Red.. 

Wisconsin  . 

92 

Yellow... 

Pennsylvania 

107 

Silver 

Wisconsin 

56 

Yellow  

Wisconsin  . 

103 

Sugar 

Indiana 

104 

Buckeye,  yellow 

Tennessee 

g 

Sugar 

Pennsylvania 

108 

Buckthorn,  cascara  

Oregon  

84a 

Sugar  .  . 

Wisconsin...  . 

124 

Butternut  

Tennessee 

27 

Oak- 

Butternut.  

Wisconsin  

21 

Bur  

Wisconsin  

125 

Chinquapin,  western 

Oregon 

46b 

California  bla^k 

California 

80 

Cherry: 

Canyon  live 

California  

163 

Black  

Pennsylvania 

72 

Chestnut 

Tennessee 

121 

Wild  red.... 

Tennessee 

24 

Cow 

133 

Chestnut  

Maryland  .. 

46 

Laurel 

Louisiana 

116 

Chestnut  

Tennessee 

40 

Post 

Arkansas 

130 

Cottonwood,  black  

Washington.  . 

6 

Post.    . 

Louisiana 

137 

Cucumber  tree  

Tennessee 

59 

Red 

Arkansas 

119 

Dogwood: 

Red  

Indiana    . 

118 

Flowering  

Tennessee 

151 

Red 

Louisiana 

117 

Western  

Oregon  

125a 

Red  

Tennessee  

97 

Elder,  pale... 

Oregon 

94 

Elm: 

Lowland  Spanish 

Louisiana  

142 

Cork  

W  isconsin 

126 

Swamp  white 

150 

M  a  r  a  t  h  on 

Tanbark  

California  

115 

County. 

Water  . 

Louisiana  .  . 

111 

Cork  

W  i  s  c  o  n  s  in, 

White 

Arkansas 

132 

RuskCounty. 

White... 

Indiana  

138 

Slippery  

Indiana  

102 

White 

Louisiana, 

136 

SJppery  
White.;  

Wisconsin  
Pennsylvania 

74, 
55 

Richland 
Parish 

White  

Wisconsin  

53 

White  

Louisiana, 

131 

Greenheart  

165 

Winn  Parish 

Gum: 

Willow... 

Louisiana  

109 

Black  

Tennessee 

68 

Yellow 

Arkansas 

122 

Blue  (Eucalyptus)  .  .  . 

California. 

147 

Yellow 

Wis  consin 

105 

Cotton  

Louisiana 

76 

Osage  orange 

Indiana 

164 

Red  

Missouri 

54 

Poplar  yellow  (tulip  tree) 

35 

Hackberry  

90 

85 

flackberry  

Wis  'onsin 

78 

Sassafras 

Tennessee 

51 

Haw,  pear  

Wisconsin  

146 

Serviceberry  

Tennessee 

156 

Hickory: 

Silverbelltree 

Tennessee 

49 

Bigshellbark  

Mississippi 

135 

Sourwood 

89 

Bigshellbark  

Ohio 

154 

Suma°  staghorn 

61 

Bitternut  

Ohio  

139 

Sycamore 

63 

Mockernut  

Mississippi 

144 

Sycamore 

65 

Mockernut  

Pennsylvania 

159 

Umbrella  Fraser 

45 

Mockernut 

West  Virginia 

155 

Willow-   ' 

Nutmeg 

112 

Bla"k 

W 

Pignut... 

Mississippi 

148 

43  a 

Pignut  

Ohio 

157 

Witch  hazel 

114 

28  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

List  of  species  and  reference  numbers  for  Fig.  13 — Continued. 
CONIFERS. 


Species. 

Locality. 

Refer- 
ence 
No. 

Species.  • 

Locality. 

Refer- 
ence 
No. 

Cedar: 
Incense     

California..  .. 

26 

Pine  —  C  ontinued  . 
Lodgepole 

40a 

Western  red 

Montana 

2 

Western  red... 

Washington  .. 

10 

White  

Wis  ^onsin 

1 

Lodgepole 

34 

Cypress,  bald  

Louisiana  .  . 

62 

Longleaf 

Florida 

123 

Douglas  fir  

California  .  . 

45a 

Longleaf 

113 

Douglas  fir  

Oregon 

67a 

Douglas  fir 

W  ashington 

46a 

Longleaf 

96 

Douglas  fir 

C  h  e  h  a  1  i  s 
County. 
W  a  s  hington 

75 

Tangipahoa 
Parish. 

QK 

LewisOounty. 

Norway 

Wisconsin 

57 

Douglas  fir  

Washington, 

67 

Pitch  

Tennessee 

71 

and  Oregon. 

Pond 

Florida 

86 

Douglas  fir  .  .     . 

Wyoming 

48 

Shortleaf 

77 

Sugar  .     .  . 

Ca'ifornia 

22 

Alpine  

Colorado 

4 

Table  Mountain 

Tennessee 

82 

Amabilis 

Oregon 

39 

Western  white 

42 

Amabiiis  

Washington. 

18 

Western  yellow 

Arizona 

19 

Balsam 

Wis  ""onsin 

14 

Western 

37 

Grand 

Montana 

36 

Western 

41 

Nobie  

Oregon 

16 

Western 

Montana 

32 

White 

California 

17 

White 

Wisconsin 

25 

Hemlock: 

Redwood     

C  ali  f  ornia, 

28 

Black  

Montana 

47 

Albion. 

Eastern 

Tennessee 

52 

Redwood 

California 

13 

Eastern  

Wis  ^onsin 

15 

Korbel. 

Western 

Washington 

50 

Spruce* 

Larch,  western  

Montana  

84 

Engelmann  

Colorado, 

8 

Larch,  western 

Washington 

64 

Grand 

Pine: 
Cuban     

Florida 

127 

Engelmann  

County. 
Colorado, 

3 

Jack 

Wisconsin 

43 

San  Miguel 

Jeffrey  

California  

33 

County. 

Loblolly     .  . 

Florida 

88 

Red  

New     Hamp- 

44 

Lodgepole 

Colorado 

31 

shire. 

Lodgepole  

Montana, 

35a 

Red... 

Tennessee  

29 

G  a  1  la  tin 

White  . 

New  Hamp- 

7 

Lodgepole  

County. 
Montana, 

41a 

White  

shire. 
Wisconsin  

38 

Granite 
County 

Tamarack  
Yew,  western     

Wisconsin  
Washington... 

81 
134 

Stock  should  be  cut  up  into  as  small  sizes  as  is  practicable  before 
kiln  drying.  Large  pieces  usually  check  severely  because  the  outer 
portion  dries  and  shrinks  considerably  faster  than  the  inner  core, 
which  always  dries  slowly.  Timbers  which  contain  the  pith  and 
which  are  to  be  cut  into  smaller  sizes  later  should  at  least  be  cut 
through  the  pith  once,  or  better,  be  quartered  before  being  stored  away. 
This  will  avoid  the  large  checks  which  are  commonly  produced  in  the 
seasoning  of  timbers  containing  the  pith  by  reason  of  the  tangential 
shrinkage  being  greater  than  the  radial  shrinkage. 

RULES    FOR   PILING   LUMBER    AND   TIMBERS . 

1.  The  foundations  should  be  strong,  solid,  and  durable,  preferably 
concrete  piers  with  inverted  rails  or  I  beams  for  skids;    if    this   is 
impracticable,  creosoted  or  naturally  durable  wooden  timbers  should 
be  used. 

2.  Each  foundation  should  be  level. 


INFORMATION  FOB  INSPECTORS  OF  AIRPLANE   WOOD. 


29 


3.  The  foundations  should  not  be  over  4  feet  apart  for  lumber,  but 
may  be  farther  apart  for  larger  timbers.     For  woods  which  waro 
easily  or  for  stock  less  than  1  inch  in  thickness  foundations  should 
not  be  over  3  feet  apart. 

4.  If  the  piles  are  in  the  open,  they  should  have  a  slope  from  front, 
to  rear  of  1  inch  for  every  foot  in  length. 


PERCENT   OF  SHRINKAGE    IN  VOLUME 
o>  a  o  i5 


\ 

x 

\ 

\ 

. 

s 

X 

° 

• 

v> 

V 

in 

5 

\ 

D 

X 

m 

* 

\ 

0 

•n 

39 

» 

. 

\ 

< 

n 

Z 
X 

H 
m 

V 

0 

5 

* 

s^ 

» 

m 

> 

\ 

< 

H 

^i 

X 

s, 

H 

0 

. 

^ 

X 

-< 

s^ 

</> 

s^ 

-o 

\ 

:  8 

^ 

*  ""* 

s 

0    0* 

V 

*lS    0 

\ 

3  o  2 

s^ 

D 

US  5 

\ 

0 

"*  ^'u, 

\ 

5  T 

\ 

3 

'  i 

S, 

<  5 

n 

> 

'n 

I  2 

< 

S, 

) 

b 

K     *  . 

b 

\ 

"  Sw 

5 

c 

Q 

-< 

g 

s 

^ 

"*  O 

c 

c 

)\ 

1° 

0 

S 

3  ^> 

. 

1 

S 

f 

°f 

f) 

2_ 

n 

^ 

i  n 

O 

^ 

Q, 

^ 

"   C 

> 

<X> 

In 

— 

5.  The  foundations  should  be  sufficiently  high  to  allow  the  free 
circulation  of  ah-  underneath  the  piles;  and  weeds  or  other  obstruc- 
tions to  circulation  should  be  removed. 

6.  Boards  of  equal  length  should  be  piled  together  with  no  free 
unsupported  ends. 


84727—19 3 


30  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

7.  A  space  of  about  three-fourths  of  an  inch  should  be  left  between 
boards  of  each  layer,  and  from  1  to  2  inches  between  timbers  of  each 
layer. 

8.  The  stickers  should  be  of  uniform  thickness,  preferably  seven- 
eighths  of  an  inch  for  1-inch  lumber  and  1^  inches  for  thicker  stock. 

9.  Stickers  should   be  placed  immediately   over   the  foundation 
beams  and  kept  in  vertical  alignment  throughout  the  piles.     Their 
length  should  be  slightly  in  excess  of  the  width  of  the  pile. 

10.  The  front  and  rear  stickers  should  be  flush  with  or  protrude 
slightly  beyond  the  ends  of  the  boards. 

KILN-DRYING  OF  WOOD. 

ADVANTAGES    OF    KILN-DRYING. 

The  chief  objects  of  kiln-drying  airplane  stock  are  (a)  to  eliminate 
most  of  the  moisture  in  green  or  partly  dried  stock  more  quickly 
than  can  be  done  in  air-drying,  and  (&)  to  reduce  the  moisture  con- 
tent of  the  wood  below  that  attained  in  ordinary  air-drying,  so  that 
no  more  drying  with  consequent  checking,  warping,  and  opening  up 
of  seams  will  occur  after  the  wood  is  in  place.  Other  advantages 
incident  to  kiln-drying  are  that  a  smoother  surface  can  be  obtained 
on  kiln-dried  stock,  that  glues  will  hold  better,  and  that  kiln-dried 
stock  will  not  shrink  and  swell  with  changes  in  atmospheric  humidity 
as  much  as  air-dried  material.  This  last  feature  is  extremely  impor- 
tant in  airplane  construction  since  it  reduces  the  loosening  of  metal 
parts  and  fittings. 

THE    ELIMINATION    OF   MOISTURE    FROM    WOOD. 

Green  lumber  may  contain  from  about  one-third  to  two  and  one- 
half  tunes  its  oven-dry  weight  of  water.  Expressed  in  percentage, 
this  is  from  33  J  to  250  per  cent  moisture  based  on  the  oven-dry  weight. 
The  moisture  content  of  green  lumber  varies  with  the  species,  the 
position  in  the  tree,  whether  heartwood  or  sap  wood,  the  locality  in 
which  the  tree  grew,  and  the  drying  which  has  taken  place  since  the 
tree  was  cut.  As  a  rule  sapwood  contains  more  moisture  than  heart- 
wood,  although  in  some  species,  especially  in  butt-logs,  the  heartwood 
contains  as  much  moisture  as  the  sapwood.  Thoroughly  air-dried 
lumber  may  contain  from  about  10  to  20  per  cent  moisture  for  inch 
stock,  and  more  for  thicker  material. 

Much  of  the  moisture  in  green  wood  is  contained  in  the  cell  cavities 
(like  honey  in  a  comb),  and  the  rest  is  absorbed  by  the  cell  walls. 
When  wood  is  drying  the  moisture  first  leaves  the  cell  cavities  and 
travels  along  the  cell  walls  to  the  surface,  where  it  is  evaporated. 


INFORMATION   FOR   INSPECTORS  OF   AIRPLANE   WOOD.  31 

When  the  cell  cavities  are  empty  but  the  cell  walls  are  still  saturated 
a  critical  point  is  reached  known  as  the  fiber  saturation  point.  Wood 
does  not  shrink  or  increase  in  strength  while  seasoning  until  it  has 
dried  below  the  fiber  saturation  point,  which  usually  ranges  between 
25  and  30  per  cent  moisture  but  may  be  less  or  more,  and  in  spruce 
usually  is  between  30  and  35  per  cent.  This  has  an  important  bearing 
on  the  drying  operation,  since  no  case-hardening,  checking,  or  warping 
can  occur  so  long  as  the  moisture  content  is  above  the  fiber  saturation 
point  in  all  parts  of  the  stick. 

In  practice  the  stock  should  be  dried  to  a  moisture  content  slightly 
less  than  it  will  ultimately  have  when  in  use.  This  may  be  as  low 
as  6  per  cent  to  10  per  cent  for  interior  work  and  not  so  low  for  wood 
to  be  exposed  to  weather. 

Two  steps  are  necessary  in  the  drying  of  lumber — (a)  the  evapora- 
tion of  moisture  from  the  surface  and  (&)  the  passage  of  moisture 
from  the  interior  to  the  surface.  Heat  hastens  both  these  processes. 
For  quick  drying,  as  high  a  temperature  should  be  maintained  in 
the  kiln  as  the  wood  will  endure  without  injury.  Some  woods 
(especially  coniferous  woods)  will  endure  higher  temperatures  than 
others.  The  General  Specifications  for  Kiln-Drying  Airplane  Stock 
(No.  20500-A)  give  the  temperatures  at  which  a  kiln  should  be  oper- 
ated to  prevent  injury  to  lumber  to  be  used  for  airplanes. 

The  lumber  in  a  kiln  is  heated  and  evaporation  is  caused  by  means 
of  hot  air  passing  through  the  piles.  To  insure  proper  drying  through- 
out the  piles  a  thorough  circulation  of  air  is  necessary.  The  lumber 
must  be  properly  piled  and  the  kiln  constructed  so  as  to  make  the 
necessary  circulation  possible. 

Dry  hot  air  will  evaporate  the  moisture  from  the  surface  more 
rapidly  than  it  can  pass  from  the  interior  to  the  surface,  thus  pro- 
ducing uneven  drying  with  consequent  damaging  results.  To  prevent 
excessive  evaporation  and  at  the  same  time  keep  the  lumber  heated 
through,  the  air  circulating  through  the  piles  must  not  be  too  dry; 
that  is,  it  must  have  a  certain  humidity.  Specification  No.  20500-A 
gives  the  proper  humidities  at  which  to  operate  the  kiln  for  .drying 
airplane  stock. 

THREE    ESSENTIAL    QUALITIES    OF   THE    DRY-KILN. 

The  merits  of  any  method  of  drying  airplane  woods  depend  upon 
the  extent  to  which  it  affects  the  mechanical  properties  of  the  stock 
and  upon  the  uniformity  of  the  drying.  In  order  that  complete 
retention  of  properties  and  uniform  drying  may  be  guaranteed,  it 
is  essential  that  the  circulation,  temperature,  and  humidity  of  the 
air  be  adequately  controlled.  In  this  connection  circulation  does 
not  mean  the  passage  of  air  through  flues,  ducts,  or  chimneys,  but 


32  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

through  the  piles  of  lumber,  and  the  terms  temperature  and  humid- 
ity control  apply  to  the  air  within  the  piles  of  lumber  in  the  kiln. 

Control  of  air  circulation  involves  rate  or  speed,  and  uniformity. 
A  uniform  passage  of  air  through  all  portions  of  the  piles  of  lumber 
is  the  most  essential  quality  in  a  kiln.  If  the  circulation  can  be 
made  both  uniform  and  rapid,  all  portions  of  the  pile  will  dry  quickly 
and  at  the  same  rate.  Furthermore,  uniform  and  rapid  circulation 
of  air  are  necessary  before  the  control  of  temperature  and  humidity 
within  the  piles  of  lumber  is  possible. 

When  unsaturated  air  at  any  given  temperature  enters  a  pile  of 
lumber  containing  moisture,  it  exchanges  heat  for  moisture,  is  cooled, 
and  rapidly  approaches  saturation.  With  green  wood  and  a  sluggish 
circulation,  the  cooling  is  very  rapid.  The  rate  of  cooling  decreases 
as  the  lumber  dries;  and  if  the  circulation  is  increased,  the  loss  of 
heat  in  passing  through  the  pile  is  less.  So  if  the  air  moves  rapidly 
through  certain  parts  of  the  piles  and  slowly  through  others,  the* 
different  parts  of  the  piles  will  be  at  different  temperatures.  The 
temperature  of  the  air  within  the  lumber  can  not  be  maintained  at 
any  given  value  unless  the  circulation  of  air  is  uniform  at  all  points 
in  the  pile.  Even  though  the  air  moves  at  uniform  speed  from  one 
side  of  a  pile  of  lumber  to  the  other,  if  the  speed  is  too  slow  the  air 
loses  its  heat  and  approaches  saturation  rapidly.  In  general,  a  wide 
variation  in  the  temperature  of  the  lumber  in  different  parts  of  the 
kiln  is  proof  of  very  uneven  or  slow  circulation.  Inadequate  circula- 
tion and  temperature  control  render  the  control  of  humidity  and 
uniform  drying  impossible. 

Humidity  is  of  prime  importance  because  the  rate  of  drying  and 
the  prevention  of  checking  and  case-hardening  are  directly  dependent 
thereon.  It  is  generally  true  that  the  surface  of  the  wood  should 
not  dry  more  rapidly  than  the  moisture  transfuses  from  the  center 
to  the  surface.  The  rate  of  evaporation  must  be  controlled  and  this 
can  be  done  by  means  of  the  relative  humidity.  Stopping  the 
circulation  to  obtain  a  high  humidity  or  increasing  the  circulation 
by  opening  ventilators  to  reduce  the  humidity  is  not  good  practice. 
Humidity  should  be  raised,  if  necessary,  to  check  evaporation,  with- 
out reducing  the  circulation. 

DEFECTS   DUE   TO   IMPROPER   DRYING. 

Case-hardening  and  honeycombing. — When  the  surface  of  a  piece 
of  lumber  is  dried  more  rapidly  than  the  moisture  can  pass  to  it 
from  the  interior,  unequal  moisture  conditions  exist  in  the  lumber. 
The  moisture  in  the  outer  layers  falls  below  the  fiber  saturation 
point.  The  outer  layers,  then  tend  to  shrink  but  are  held  from 
shrinking  by  the  more  moist  interior,  which  has  not  yet  started  to 


FIG.  14.— Sections  of  case-hardened  western  larch  boards.    Nos.  1  and  2  are  original  sections. 
Nos.  3  to  8  are  resawed  sections  showing  cupping.    No.  9  is  one-side  surfaced. 


INFORMATION  FOB  INSPECTORS  OF  AIRPLANE   WOOD.  33 

shrink;  so  the  surface  either  checks  or  dries  in  a  stretched  condition, 
usually  both.  Later,  as  the  interior  dries  it  also  tends  to  shrink 
normally,  but  in  turn  is  held  by  the  outside,  which  has  become 
"set"  or  " case-hardened."  Consequently,  the  interior  dries  under 
tension,  which  draws  the  outer  layers  together  closing  up  all  checks 
and  producing  compression.  Case-hardened  lumber,  when  resawed, 
will  invariably  cup  toward  the  inside  if  the  interior  of  the  lumber 
is  dry.  (Fig.  14.)  If  the  tension  in  the  interior  of  the  wood  is 
severe  enough,  it  may  produce  radial  checks  which  do  not  extend 
to  the  surface.  Wood  with  such  checks  is  said  to  be  honeycombed 
or  hollow-horned.  (Fig.  15.)  Case-hardening  and  honeycombing 
can  practically  be  prevented  by  regulating  the  humidity  so  that  the 
evaporation  from  the  surface  does  not  take  place  too  rapidly. 

If  wood  becomes  case-hardened  in  kiln  drying  it  may  be  brought 
back  to  normal  condition  by  steaming,  provided  that  checks  and 
cracks  have  not  developed.  Steaming  softens  the  outer  fibers  and 
relieves  the  stresses  caused  by  the  contraction  of  the  outer  shell. 
Care  must  be  taken  not  to  steam  wood  which  has  checked  or  honey- 
combed from  case-hardeneing  enough  to  part  the  fibers  and  weaken 
the  piece.  Steaming  will  close  up  the  cracks  but  will  not  restore 
the  strength  of  the  piece.  It  will  be  much  harder  to  detect  cracks 
and  checks  due  to  case-hardening  if  they  have  been  closed  up  again 
by  steaming. 

Collapse. — Collapse  is  abnormal  shrinkage  causing  grooves  to  appear 
in  the  surface  of  the  lumber  or  a  general  distortion  of  the  surface. 
(Fig.  16.)  It  is  produced  when  wet  lumber  is  dried  at  too  high  a 
temperature.  The  heat  and  moisture  cause  the  cell  walls  to  become 
soft  and  plastic.  As  the  water  leaves  the  cell  cavities  the  moist  cell 
walls  are  drawn  together  if  no  air  is  present.  This  causes  the  cells  to 
flatten,  and  a  general  reduction  in  the  cross  section  takes  place. 
Collapse  occurs  especially  in  such  woods  as  western  red  cedar,  red- 
wood, white  oak,  and  others  which  readily  become  soft  and  plastic 
when  hot  and  moist.  It  can  be  avoided  by  not  allowing  the  tem- 
perature to  rise  too  high  while  the  wood  is  still  moist  (at  or  above  the 
fiber  saturation  point). 

Brashness. — High  temperature  treatments  of  all  kinds,  whether 
steam  or  hot  air,  are  injurious  to  lumber,  causing  it  to  turn  darker 
and  become  brash.  The  injuries  thus  sustained  increase  with  the 
temperature  and  length  of  time  the  wood  is  exposed  to  such  severe 
conditions.  No  definite  rule  can  be  laid  down  as  to  what  conditions 
of  temperature  wood  will  endure  without  becoming  brash.  If  the 
temperature  prescribed  in  Specification  No.  20500-A  is  not  exceeded, 
no  difficulty  will  be  experienced  in  this  respect. 


34  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

TESTING   OF    KILNS    FOR   DRYING   AIRPLANE    STOCK. 

Kinds  of  tests. — Some  kilns  are  better  adapted  than  others  to  dry- 
ing airplane  stock.  In  many  cases  present  trouble  can  be  remedied 
when  once  understood  by  the  operator.  As  assurance  that  the  stock 
will  be  acceptable,  however,  careful  check  must  be  kept  at  all  times 
on  every  kiln  used  for  drying  airplane  woods.  The  following  tests 
will  aid  the  inspector  in  keeping  check  on  any  kiln: 

I.  Preliminary  tests: 

a.  Initial  moisture  conditions  in  the  lumber. 
6.  Preparation  and  placing  of  samples. 

c.  Initial  weights  and  placing  of  whole  pieces. 

d.  Determination  of  direction,  uniformity,  and  rate  of  air  circulation. 

e.  Location  and  calibration  of  instruments. 
II.  Current  tests: 

a.  Determination  of  current  temperatures. 

b.  Determination  of  current  humidities. 

c.  Determination  of  circulation. 

d.  Weighing  of  samples  and  determination  of  current  moisture  conditions. 
III.  Final  tests: 

a.  Average  kiln-dry  moisture  condition  of  samples. 
6.  Distribution  of  moisture  in  the  kiln-dry  samples. 

c.  Determination  of  case-hardening  in  kiln-dry  samples. 

d.  Average  kiln-dry  moisture  condition  of  whole  pieces. 

e.  Calculation  of  initial  moisture  condition  of  whole  pieces. 
/.  Distribution  of  moisture  in  kiln-dry  whole  pieces. 

g.  Distribution  of  case-hardening  in  kiln-dry  whole  pieces. 
h.  Determining  the  effects  of  the  process  on  the  toughness  and  strength  of  the 
kiln-dry  stock. 

Instruments. — In  making  these  tests  the  following  instruments  and 
material  are  recommended : 

1  sensitive  equal  arm  balance  (capacity  0.1  to  250  grams). 

1  drying  oven  in  which  the  air  can  be  heated  to  and  held  at  212°  F. 

1  can  of  asphalt  paint  and  a  brush. 

1  sensitive  platform  scale  (capacity  0.01  to  250  pounds). 

1  electric  flash  light  (lantern  type  recommended). 
12  packages  of  punk  sticks. 

3  accurate  standardized  ordianry  glass  thermometers  (60°  to  230°  F.  by  2°  intervals). 

2  accurate  standardized  glass  wet  and  dry  bulb  hygrometers  with  extra  wicks  (60° 
to  230°  F.  by  2°  intervals). 

Access  to  a  laboratory  equipped  with  machines  for  making  impact,  static  bending, 
hardness,  compression  parallel  to  the  grain,  and  other  tests. 
Waxed  or  oiled  paper. 

METHODS. 

7.  Preliminary  tests. — (a)  Initial  moisture  condition:  Select  at  least 
three  representative  pieces  for  each  5,000  board  feet  of  stock  to 
be  dried.  Cut  about  2  feet  from  one  end  of  each.  Then  cut  a 
1-inch  section,  a  24-inch  sample,  and  a  second  1-inch  section  in  suc- 
cession. Immediately  weigh  the  two  1-inch  sections  to  an  accuracy 


FIG.  16.— End  view  of  1-inch  boards  of  western  red  cedar  dried  with  and 
without  collapse. 


INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD.  35 

of  one-tenth  of  1  per  cent.  Mark  the  initial  weights  on  the  sections, 
and  dry  them  to  constant  weight  in  the  oven  heated  to  212°  F. 
Re  weigh  them  to  the  same  accuracy  and  determine  the  per  cent 
initial  moisture  content  of  the  samples  from  the  formula : 

initial  weight— oven  dry  weight.  .. 
Per  cent  initial  moisture  content=  oven  dry  weight 

(6)  Preparation  and  placing  of  samples:  Immediately  after  cutting 
the  24-inch  samples  described  under  (a)  paint  the  ends  of  the  sam- 
ples with  a  heavy  coat  of  asphalt  paint.  Then  weigh  them  sepa- 
rately on  the  platform  scale  to  an  accuracy  of  one-tenth  of  I  per  cent. 
Mark  the  initial  weights  on  the  samples  and  place  them  in  the  piles 
so  as  to  come  under  the  most  severe,  least  severe,  and  average  drying 
conditions,  and  so  as  to  be  subjected  to  the  same  drying  conditions 
as  the  adjacent  pieces.  Where  the  circulation  of  air  is  vertical,  place 
samples  near  the  tops,  centers,  and  bottoms  of  the  piles;  and  where 
the  circulation  is  lateral  place  them  near  the  sides  where  the  air 
enters  and  leaves  the  piles  and  near  the  centers  of  the  piles. 

(c)  Initial  weights  and  placing  of  whole  pieces:  In  addition  to  test 
(a),  it  is  desirable  to  select  several  representative  whole  pieces  of 
stock  and  weigh  them  to  an  accuracy  of  one-tenth  of  1  per  cent  on 
the  platform  scale.     Mark  the  weights  on  the  pieces  and  place  them 
at  various  points  near  the  tops,  edges,  bottoms,  and  centers  of  the 
piles. 

(d)  Determination  of  the  direction,  uniformity,   and  rate  of  air 
circulation:    In  order   to  insure  correct  placing  of  samples,  whole 
pieces,  and  instruments  it  is  necessary  that  the  direction  of  the  circu- 
lating air  be  known.     To  determine  this  light  a  few  punk  sticks,  take 
the  flash  light,  enter  the  kiln,  close  the  door,  and  determine  the  direc- 
tion, uniformity,  and  rate  of  motion  of  the  circulating  air  in  the  space8 
around  the  piles  and  through  the  piles  by  observing  the  smoke  from 
the  burning  punk. 

(e)  Location  and  calibration  of  instruments:    Having  determined 
the  direction  in  which  the  air  passes  through  the  piles,  place  tbe 
bulb  of  the  recording  thermometer  in  contact  with  a  standardized 
glass  thermometer  close  to  the  pile  at  the  center  of  the  side  where  the 
air  enters  the  pile.     If  the  circulation  is  up  through  the  pile,  place 
the  thermometer  bulbs  close  under  the  bottom  center;  if  it  is  down 
through  the  lumber,  place  the  bulbs  close  to  the  top  center;   and  if 
the  air  moves  through  the  pile  laterally,  place  the  bulbs  close  to  the 
center  of  the  side  where  the  air  enters  the  pile.     It  is  also  desirable 
to  know  the  variation  of  temperature  in  different  parts  of  the  piles 
and  kiln.     To  determine  this  variation,  place  several  of  the  standard- 
ized thermometers  in  the  tops,  bottoms,  edges,  and  centers  of  the 
piles  and  at  different  points  in  the  kiln.     In  order  to  calibrate  a 
recording  thermometer,  place  the  bulb  in  contact  with  a  standardized 


36  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

glass  thermometer  in  the  kiln  and  adjust  the  stylus  until  it  agrees 
with  the  glass  thermometer.  The  temperature  must  not  be  fluctuat- 
ing as  is  often  the  case  where  it  is  controlled  by  a  thermostat.  It  is 
best  to  use  a  steady  steam  pressure  in  the  heating  pipes  while  cali- 
brating instruments.  Never  attempt  to  calibrate  a  recording  ther- 
mometer out  of  its  place  in  the  kiln. 

To  determine  humidity,  place  the  standardized  glass  wet  and  dry 
bulb  hygrometer  near  the  bulb  of  the  recording  thermometer,  so  as 
to  indicate  the  humidity  of  the  air  entering  the  piles  at  the  tops,  bot- 
toms, or  edges  as  the  case  may  be. 

II.  Current  tests. — (a)  Determination  of  current  temperatures:  If 
any  part  of  a  pile  is  exposed  to  direct  radiation  from  the  heating 
pipes,  place  a  thermometer  near  the  side  so  exposed.  This  will  indi- 
cate whether  or  not  any  part  is  subject  to  higher  temperature  than 
that  indicated  by  the  recording  instrument.  If  possible,  allow  no 
direct  radiation  on  the  lumber.  The  temperature  of  the  air  entering 
the  piles  must  be  known  at  all  times,  preferably  by  means  of  recording 
thermometers  with  extension  bulbs  which  have  been  calibrated  in 
place  as  directed  under  I  (e). 

The  temperatures  in  the  tops,  bottoms,  edges,  and  centers  of  the 
piles  and  at  different  points  in  the  kiln  should  be  determined  occa- 
sionally by  using  standardized  thermometers  located  as  directed 
under  I  (e). 

(b)  Determination  of  current  humidities:  Never  attempt  to 
determine  the  relative  humidity  of  the  air  where  the  bulbs  of  the 
hygrometer  are  exposed  to  direct  radiation.  Where  direct  radiation 
may  take  place,  it  is  necessary  to  shield  the  hygrometer  from  the 
heating  pipes  before  readings  are  taken.  The  relative  humidity  of 
the  air  entering  the  piles  must  be  indicated  at  all  times  by  means  of 
standardized  glass  wet  and  dry  bulb  hygrometers  placed  as  directed 
under  I  (e).  Before  reading  the  hygrometer,  fan  the  bulbs  briskly 
for  about  a  minute,  so  that  they  are  at  the  temperature  of  the  air  in 
the  kiln.  An  air  circulation  of  at  least  15  feet  per  second  past  the 
wet  bulb  is  necessary  for  an  accurate  humidity  reading.  The  wick 
should  be  of  thin  silk  or  linen  and  it  must  be  free  from  oil  or  dirt  at 
all  times.  It  should  come  into  close  contact  with  as  much  of  the 
bulb  as  possible.  Knowing  the  correct  wet  and  dry  bulb  hygrometer 
readings,  the  .relative  humity  may  be  determined  from  the  humidity 
diagram  in  figure  17. 

Relative  humidity  is  shown  on  the  horizontal  scale  and  Fahrenheit 
temperature  on  the  vertical  scale.  The  curves  running  from  the  top 
left  to  the  bottom  right  part  of  the  chart  are  for  various  differences 
in  the  wet  and  dry  bulb  readings.  The  curves  are  numbered  near  the 
center  of  the  chart  above  the  heading  "  (t — tj  degrees  Fahrenheit." 


INFORMATION  FOB  INSPECTORS  OF  AIRPLANE   WOOD.  37 

10          20          30          40          50  60          70  80        '  90          100 


210 


220 


200 


210 


220 


10          20  30          40          50  60  70  80  90  100 

RELATIVE    HUMIDITY-PER   CENT 
FIG.  17. 


38  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

To  get  the  relative  humidity,  follow  the  curve  which  is  numbered  to 
correspond  to  the  difference  of  the  wet  and  dry  bulb  readings  till  it 
intersects  the  horizontal  line  numbered  to  correspond  to  the  dry 
bulb  reading.  Directly  below  this  intersection  in  a  vertical  line  will 
be  found  the  relative  humidity  on  the  bottom  scale.  Example:  Dry 
bulb  reading  120;  wet  bulb  reading  113;  difference  7.  Curve  7 
intersects  horizontal  line  120  at  vertical  line  79.  Kelative  humidity 
is  79  per  cent. 

When  the  humidity  is  desired  in  a  Tiemann  kiln  use  the  set  of  curves 
running  from  the  top  right  to  the  bottom  left  part  of  the  chart. 
Locate  the  lower  of  the  two  thermometer  readings  on  the  scale  at 
the  right  of  the  chart.  This  is  the  reading  of  the  thermometer  in  the 
baffle  box.  Follow  along  parallel  to  the  nearest  curve  till  the  hori- 
zontal line  is  crossed  whose  number  is  the  higher  thermometer 
reading.  Vertically  below  this  point  of  intersection  on  the  lower 
scale  will  be  found  the  relative  humidity.  Example:  Baffle  ther- 
mometer reading,  112°;  flue  thermometer  reading  120°.  Start  at  112 
on  right-hand  scale,  follow  parallel  to  curve  28  till  horizontal  line 
120  is  crossed.  This  point  falls  on  vertical  line  80.  Relative  humidity 
is  80  per  cent. 

(c)  Determination  of  circulation:  During  each  drying  operation 
the  circulation  of  the  air  should  be  tested  several  times  as  under  I  (d) . 
As  the  lumber  becomes  drier,  it  has  less  cooling  effect  on  the  air,  and 
this  may  change  the  circulation  in  the  kiln.     If  this  occurs,  corre- 
sponding changes  in  the  location  of  instruments  should  be  made. 

(d)  Weighing  of  samples  and  determination  of  current  moisture 
condition:  The   24-inch  samples,   placed   as   directed  under  I    (6), 
should  be  weighed  daily  to  an  accuracy  of  one-tenth  of  1  per  cent 
on  the  platform  scale.     From  test  I  (a),  the  initial  moisture  contents 
of  these  samples  are  known.     Their  initial  weights  were  deterimned 
by  test  I  (6).     Knowing  their  initial  moisture  contents  and  weights, 
their  oven-dry  weights  may  be  computed  from  the  formula: 

.  ,  .  ,  -  Initial  weight v 

Oven-dry  weight  of  sample =ioo+5itSO^Stiire  contentX- 

Having  the  calculated  oven-dry  weights  and  daily  weights  of  the 
samples,  their  current  moisture  contents  may  be  computed  from  the 

formula: 

current  weight— oven-dry  weight 
Current  moisture  content  of  sample  =  oven-dry  weight 

Therefore,  since  the  samples  were  cut  from  representative  stock, 
the  drying  rate  of  the  material  is  known  currently. 

III.  Final  tests. — (a)  Average  kiln-dry  moisture  condition  of  sam- 
ples: When  the  current  moisture  contents  of  the  samples  indicate 
that  the  material  is  dried  to  the  required  point,  three  1-inch  sections 
are  cut  from  the  center  of  each  sample.  One  section  from  each 


INFORMATION  FOR*  INSPECTORS  OF  AIRPLANE   WOOD.  39 

sample  is  used  to  determine  the  average  kiln-dry  moisture  content 
of  each  sample  by  the  method  of  test  I  (a).  This  test  must  be  made 
immediately  after  sawing. 

(5)  Distribution  of  moisture  in  kiln-dry  samples:  A  thin  shell 
(about  one-fourth  inch)  is  split  from  the  four  outer  surfaces  of  the 
second  1-inch  section  cut  from  each  sample.  The  outsides  and 
centers  are  tested  for  moisture  content  separately  and  immediately 
after  sawing  by  the  method  of  I  (a).  The  results  of  this  test  show 
the  distribution  of  moisture  in  cross  sections  of  the  samples.  The 
difference  between  the  moisture  contents  of  the  outer  shells  and  the 
centers  shows  whether  or  not  the  distribution  is  sufficiently  uniform 
across  the  sections. 

(c)  Determination  of  case-hardening  in  kiln-dry  samples :  The  first 
indication  of  case-hardening  is  surface  checking.     The  next  sign  of 
case-hardening   is    honeycombing   or   interior    checking    along    the 
medullary   rays.     This    defect   can   not    always   be  ^detected  by  a 
superficial  inspection.     It  is  necessary  to  cut  the  stock  to  discover  it. 
Occasionally  it  is  evidenced  by  a  bulging  of  the  surface  over  the 
honeycombed    part.     Often    neither    of    these    defects    is    present. 
In  this  case  the  third  1-inch  section  from  each  sample  is  resawed 
two  or  three  times  from  one  end  down  to  within  about  half  an  inch 
of  the  other  end  (see  Fig.  14).     If  the  material  is  case-hardened  and 
dry,  it  will  pinch  the  saw;  if  it  is  not  dry  at  the  time  of  sawing,  the 
cupping  of  the  outer  prongs  will  increase  upon  further  drying.     If 
the  kiln-dried  samples  show  casehardening,  the  material  should  be 
steamed  until  the  resawed  sections  do  not  pinch  the  saw  in  this  test. 

(d)  Average   kiln-dry   moisture   condition   of   the   whole   pieces: 
When  the  kiln  is  unloaded,  the  whole  pieces  from  different  parts  of 
the  piles  and  kiln  are  weighed  and  then  cut  as  follows :  Remove  about 
2  feet  from  one  end  and  then  cut  off  three  1-inch  sections.     The 
average  kiln-dry  moisture  contents  of  the  whole  pieces  are  deter- 
mined from  one  section  as  in  test  III  (a).     The  other  sections  are  used 
as  stated  in  III  (f)  and  III  (#). 

(e)  Calculation  of  initial  moisture  condition  of  whole  pieces :  From 
the  kiln-dry  weights  and  kiln-dry  moisture  contents  of  the  whole 
pieces,  their  oven-dry  weights  may  be  computed  from  the  formula: 

Oven-dry  weight  of  whole  piece3  =f7T          ^In-dry  weight 

100+kiln-dry  moisture  content 

Knowing  the  initial  weights  and  oven-dry  weights  of  the  whole 
pieces,  their  initial  moisture  contents  are  computed  from  the  formula: 

Initial  moisture  content  of  whole  piece  ^Initial 

Therefore  the  initial  and  kiln-dry  moisture  conditions  of  the 
samples,  whole  pieces,  and  the  average  stock  are  known. 


40  INFORMATION"   FOR   INSPECTORS  OF 'AIRPLANE   WOOD. 

(f)  Distribution  of  moisture  in  kiln-dry  whole  pieces:  This  test  is 
a  duplicate  of  test  III  (&). 

(g)  Determination   of   case-hardening  in   kiln-dry   whole   pieces: 
This  test  is  a  duplicate  of  test  III  (c). 

(h)  To  determine  the  effect  of  drying  on  the  strength  of  the  stock: 
It  is  practically  impossible  to  determine  the  effect  of  the  process  of 
drying  on  the  properties  of  the  stock  by  inspection  unless  some 
visible  defect  has  developed.  This  is  not  usual,  and  as  the  inspector 
can  not  always  resort  to  mechanical  tests  he  should  be  able  to  show 
from  his  operation  records  that  conditions  in  the  kiln  have  been  kept 
within  the  specifications  recommended  as  safe  for  kiln-drying  airplane 
stock. 

TREATMENT   OF    WOOD    AFTER   REMOVAL   FROM    THE   DRY   KILN. 

Lumber  should  be  retained  for  at  least  two  weeks  after  removal 
from  the  dry  kiln  in  a  shed  or  room  where  the  conditions  are  approxi- 
mately the  same  as  in  the  shop  where  the  material  is  to  be  worked 
up.  The  necessity  for  this  will  be  understood  upon  consideration 
of  the  following  facts:  When  lumber  is  drying  in  the  kiln  the  outer 
surface  is  necessarily  somewhat  drier  than  the  interior.  In  good 
methods  of  drying  this  difference  is  a  minimum  and  in  bad  methods 
of  drying  it  is  excessive;  but  it  exists  to  a  certain  extent  in  all  methods 
of  drying.  When  the  lumber  has  been  dried  down  to  a  point  some- 
what below  the  condition  to  which  it  will  finally  come  when  exposed 
to  the  normal  shop  working  conditions,  it  will  gradually  reabsorb 
moisture  on  the  outside.  Thus,  thoroughly  kiln-dried  lumber,  if  it 
has  stood  in  an  unheated  room  for  some  time,  will  be  found  to  be 
drier  on  the  inside  than  it  is  on  the  surface,  though  the  difference 
is  likely  to  be  very  small.  Since  differences  in  moisture  content  are 
indicative  of  internal  stresses  existing  in  the  wood,  it  is  evidently 
desirable  to  have  the  moisture  distribution  as  uniform  as  possible 
before  the  lumber  is  made  up  into  finished  products;  otherwise  the 
adjustment  of  stresses,  when  the  lumber  has  been  cut  up,  will  cause 
warping,  checking,  or  other  troubles. 

Just  how  long  lumber  should  remain  in  the  shop  air  after  being 
kiln-dried  will  depend,  of  course,  upon  a  great  many  circumstances. 
Generally  speaking,  the  longer  it  remains  the  better  it  will  be,  pro- 
vided the  moisture  conditions  of  the  room  in  which  it  is  stored  are 
suitable.  The  same  kind  of  a  test  as  has  been  explained  for  case- 
hardening  occurring  in  the  dry-kiln  will  apply  as  a  test  of  the  lumber 
after  remaining  in  storage,  to  see  whether  the  internal  stresses  have 
been  neutralized. 

Even  if  case-hardening  has  been  removed  in  the  dry  kiln  by  re- 
steaming  at  the  end  of  the  drying  period,  there  may  still  exist  within 


INFORMATION   FOIl   INSPECTORS   OF   AIRPLANE   WOOD.  41 

the  lumber  slight  differences  in  moisture  content  which  will  gradu- 
ally adjust  themselves  under  proper  storage  conditions,  so  that  mate- 
rial which  has  been  steamed  before  removal  from  the  kiln  is  also  bene- 
fited by  being  allowed  to  stand  in  the  room  before  it  is  manu- 
factured. 

Ideal  conditions  for  the  storage  and  manufacturing  of  lumber 
require  regulation  of  the  humidity,  which  should  be  kept  slightly 
below  that  of  the  average  conditions  to  which  the  lumber  is  to  be 
subjected  after  it  is  put  into  service.  The  nearer  these  conditions  are 
actually  met  in  practice  the  better  are  the  results  to  be  expected, 
particularly  where  requirements  are  so  exacting  as  in  the  construc- 
tion of  airplanes. 

PUBLICATIONS   ON    KILN-DRYING   WOODS. 

1.  Government  publications: 

The  Theory  of  Drying  and  Its  Application  to  the  New  Humidity-Regulated  and  Re- 
circulating  Dry  Kiln.  U.  S.  Department  of  Agriculture  Bulletin  509.  1917.  5 
cents. 

The  Seasoning  of  Wood.  U.  S.  Department  of  Agriculture  Bulletin  552.  1917.  10 
cents. 

Principles  of  Drying  Lumber  at  Atmospheric  Pressure  and  Humidity  Diagram. 
Forest  Service  Bulletin  104.  1912.  5  cents. 

Strength  of  Wood  as  Influenced  by  Moisture.  Forest  Service  Circular  108.  1907. 
5  cents. 

NOTE.— The  above  publications  may  be  obtained  at  the  prices  indicated  from  the  Superintendent  of 
Documents,  Government  Printing  Office,  Washington,  D.  C. 

2.  Papers  Prepared  by  the  Forest  Products  Laboratory  and  published  in  various 
journals: 

Principles  of  Kiln  Drying  Lumber,  Parts  I  and  II.  .H.  D.  Tiemann,  Lumber  World 
Review,  January  25,  February  25,  1915. 

The  Kiln  Drying  of  Lumber.  H.  D.  Tiemann,  American  Lumberman,  October  30, 
1915. 

The  Circulation  in  Dry  Kilns,  Parts  I  and  II.  H.  D.  Tiemann,  Lumber  WTorld  Re- 
view, May  10,  June  10,  1916. 

Problems  in  Kiln-Drying  Lumber.  H.  D.  Tiemann,  Lumber  World  Review,  Sep- 
tember 25,  1915. 

Improvement  in  Forest  Service  Humidity-Regulated  Kiln.  H.  D.  Tiemann,  Ameri- 
can Lumberman,  September  4,  1915. 

Drying  in  Superheated  Steam.  H.  D.  Tiemann,  Lumber  World  Review,  August  10, 
1916. 

Kiln  Drying  of  Gum.    J.  E.  Imrie,  American  Lumberman,  January  22,  1916. 

Conditions  which  exist  in  Casehardened  Wood.  J.  E.  Imrie,  Southern  Lumberman, 
November  13,  1915. 

The  Casehardening  of  Wood.    H.  D.  Tiemann,  Lumber  World  Review,  July  10,  1916. 

How  to  Measure  Conditions  in  Seasoned  Lumber.  J.  E.  Imrie,  Hardwood  Record,  . 
March  10,  1917;  American  Lumberman,  March  10,  1917. 

Experiments  in  Kiln  Drying  Southern  Yellow  Pine.  J.  E.  Imrie,  The  St.  Louia 
Lumberman,  September  1,  1917;  Lumber  Trade  Journal,  September  1,  1917. 

Effects  of  Different  Methods  of  Drying  on  the  Strength  of  Wood.    H.  D.  Tiemann, 
Lumber  World  Review,  April  24,  1915. 
84727—19 4 


42  INFORMATION   FOR   INSPECTORS   OF   AIRPLANE   WOOD. 

3.  Private  publications: 
The  Kiln  Drying  of  Lumber.     H.  D.  Tiemann,  J.  B.  Lippincott  Co.,  Philadelphia. 

1917. 
Seasoning  of  Wood.    J.  B.  Wagner,  D.  Van  Nostrand  Co.,  New  York. 

CHANGES  OF  MOISTURE  IN  WOOD  WITH  HUMIDITY  OF  AIR. 

Wood  is  a  hygroscopic  material;  that  is,  it  has  the  property  of 
absorbing  moisture  from  the  air  or  surrounding  medium.  It  has 
already  been  explained  that  there  are  two  different  kinds  of  moisture 
found  in  wood,  namely,  free  water,  which  occupies  the  openings  in 
the  cell  structure  of  the  wood,  and  hygroscopic  water,  which  is  actu- 
ally taken  into  the  cell  walls  and  which  upon  being  removed  or  added 
to  wood  causes  shrinkage  or  swelling. 

There  is  a  definite  moisture  content  to  which  wood  will  eventually 
come  if  it  is  held  in  an  atmosphere  which  is  at  a  constant  humidity 
and  temperature.  The  moisture  content  of  wood  will  vary  with  the 
average  atmospheric  conditions,  also  with  the  size  of  the  material. 
Thus,  ordinary  lumber  which  is  stored  in  the  open  during  the  sum- 
mer months  for  sufficient  time  will  eventually  attain  a  moisture  con- 
tent of  from  8  to  15  per  cent,  and  wood  stored  indoors  In  a  heated 
building  will  in  time  f aH  to  about  5  or  6  per  cent  because  of  the  lower 
relative  humidity.  If  the  relative  humidity  is  constant,  an  increase 
in  temperature  decreases  the  moisture-holding  power  of  the  wood. 
However,  the  moisture  content  is  not  appreciably  affected  by  tem- 
perature within  a  range  of  25°  or  30°  F. 

Figure  18  shows  the  relation  between  the  moisture  content  of  wood 
and  the  humidity  conditions  of  the  atmosphere.  The  data  for  the 
curve  were  obtained  by  ke'eping  the  wood  at  a  constant  humidity  and 
temperature  until  no  further  change  in  moisture  occurred.  This 
curve  can  be  used  as  an  aid  in  controlling  the  moisture  conditions  of 
wood,  the  approximate  atmospheric  condition  being  known,  and  in 
determining  the  proper  humidities  for  storing  lumber  in  order  to 
secure  a  certain  moisture  content  and  uniform  material  for  use  in 
fine  wood  jointing,  propellers,  etc.  It  is  of  importance  to  have  wood 
to  be  used  for  propellers  of  uniform  moisture  content.  The  curve 
may  be  used  also  to  prepare  wood  for  use  in  a  given  locality,  such  as 
the  border  States,  where  the  humidity  is  usually  very  low.  Pro- 
pellers for  use  under  such  conditions  should  be  made  up  at  a  low 
moisture  content,  in  order  that  there  may  be  less  tendency  for 
moisture  changes  to  take  place  when  they  are  put  in  service.  It 
must  be  remembered  that  this  curve  must  not  be  used  for  dry-kiln 
work  because  of  the  fact  that  the  dry-kiln  temperatures  used  are 
higher  than  those  at  which  the  data  were  collected.  Furthermore, 
the  curve  represents  the  ultimate  moisture  content  at  a  given  tem- 
perature and  humidity,  and  in  the  case  of  large  pieces  of  wood  this 


INFOKMATTON   FOR,  INSPECTORS  OF   AIRPLANE   WOOD. 


43 


moisture  content  would  not  be  reached  for  a  long  period  of  -time. 
Kiln-drying  tends  to  reduce  the  hygroscopic  properties  of  wood, 
hence  curves  for  kiln-dried  wood  are  lower  than  the  one  given.  For 


o 


10 


10  20  30  40.  50  6O  70  80  SO  10 

FIG.  18.  —  Composite  curve  of  moisture  content  of  five  woods  at  different  humidities  and  ordinary  room  temperature. 

< 

] 

PER     GENT      MOISTURE 

\ 

\ 

\ 

^ 

\ 

\ 

\ 

CP 

X 

c 
u 

r 

5 

^ 
J 

r 

2 
C 
— 

© 

-< 

n 

r 

C 
a 

SHOWIHQ  THE.  MOISTURE  COMTEMT  OF  FIVE 
.  WOOD5  AT  DIFFERENT  HUMIDITIES  AMD 

ORDINARY  ROOM  TEMPERATURES- 
®WHITE  OAK  05ITKA  SPRUCE 

V 

c 

L 

"l 

r 

r 

T 
'3 

'r 

•  r 

"  «< 

r 

-: 
.c 

"c 

•-« 

> 

^ 

— 

3 

V 

n 

T 

} 

o" 

y 

) 

y 

0^ 

i 

^L 

H 

^ 

1 

i 

r 

C 

c 

3 

^ 

r 

3 

3 

\ 

j  * 

H 

n_ 
•>i 

n 

\ 

•» 

i 

H 

•• 

f 

1 

\ 

i 

o 

i 

-i 

V 

n 

: 

y 

1 

\ 

H~ 

j 

f_ 

H 

\ 

J\ 

\ 

I 

r 

i 

\ 

v 

y* 

S 

O' 

k 

X 

*y 

s 

u 

l 

w 

\ 

i* 

^ 

< 

1 

<? 

_j 

k 

S  > 

eo 

> 

*, 

_L 

J 

IO 


(5 


25 


example,  wood  that  had  been  dried  to  2  per  cent  moisture,  or  less,  if 
subjected  to  humidities  between  30  and  70  per  cent,  would  probably 
show  a  corresponding  moisture  content  about  1 J  to  2J  per  cent  lower 
than  in  the  curve  in  Figure  18. 


44  INFORMATION    FOR   INSPECTORS    OF   AIRPLANE    WOOD. 

GLUING  OF  WOODS. 

TESTING   OF   ANIMAL    GLUE    FOR    AIRPLANE    PROPELLERS. 

Kind  of  tests. — Chemical  analysis  has  been  found  practically  useless 
as  a  means  of  testing  glues  because  of  the  lack  of  knowledge  of  their 


FIG.  19.— Method  of  preparing  specimens  for  glue-strength  tests. 

chemical  composition.  Physical  tests  must,  therefore,  be  relied  upon. 
A  considerable  number  of  physical  tests  have  been  devised,  some  of 
which  are  important  for  one  class  of  work  and  some  for  another. 
For  judging  the  suitability  of  glue  for  high-grade  joint  work  the  tests 
considered  most  important  are  strength  (adhesiveness),  viscosity, 
jelly  strength,  odor,  keeping  qualities,  grease,  foam,  and  reaction  to 


O   •£ 


INFORMATION  FOB  INSPECTORS  OF  AIRPLANE   WOOD.  45 

litmus.  In  the  subsequent  discussion  of  these  tests,  their  application 
to  joint  glue  will  be  especially  kept  in  mind. 

Strength  tests  of  glued  joints. — Strength  tests  are  made  by  gluing 
together  two  or  more  pieces  of  wood  and  noting  the  pressure  or  pull 
required  to  break  them  apart.  Many  different  methods  of  making 
the  test  specimens  and  breaking  them  have  been  devised.  These 
depend  to  a  certain  extent  upon  the  character  of  work  expected  of 
the  glue  and  the  nature  of  the  testing  apparatus  available.  In  the 
experiments  at  the  Forest  Products  Laboratory  the'  simplest  and 
most  convenient  strength  test  found  is  to  glue  two  blocks  together 
as  shown  in  figures  19  and  20  b  and  shear  them  apart  in  a  timber- 
testing  machine.  (See  fig.  20  a  and  c.)  It  will  usually  be  found  that 
there  is  considerable  difference  in  the  values  obtained  for  the  indi- 
vidual specimens.  The  amount  of  difference,  however,  can  be  kept 
at  a  minimum  by  using  care  to  see  that  the  specimens  are  selected, 
prepared,  and  tested  under  as  nearly  the  same  conditions  as  possible. 
In  making  strength  tests  the  selection  of  the  wood  is  a  very  important 
factor.  The  species  selected  should  be  the  one  upon  which  it  is 
proposed  to  use  the  glue,  or  one  fully  as  strong.  Care  should  be  taken 
also  that  the  wood  is  above  the  average  strength  of  the  species,  in 
order  that  there  may  be  less  opportunity  for  the  wood  to  fail  before 
the  glue.  If  the  wood  is  too  weak,  the  full  strength  of  the  glue  is  not 
determined. 

No  block  should  fail  below  2,200  pounds  per  square  inch,  and  the 
average  shearing  strength  for  a  propeller  glue  should  be  at  least 
2,400  pounds  per  square  inch. 

Viscosity  test. — The  viscosity  of  a  glue  is  determined  by  allowing  a 
specified  amount  at  a  given  temperature  to  flow  through  an  orifice. 
The  time  required  is  a  measure  of  the  viscosity.  The  time  required 
for  water  to  flow  through  is  taken  as  the  standard.  In  general,  it  is 
found  that  a  glue  with  high  viscosity  is  stronger  than  one  with  a  low 
viscosity  and  will  take  more  water,  although  there  are  exceptions. 
Hide  glues,  as  a  rule,  have  higher  viscosities  than  bone  glues. 

A  number  of -different  shaped  viscosimeters  have  been  devised. 
In  the  glue  manufacturer's  laboratory,  where  many  tests  must  be 
made  each  day,  an  instrument  must  be  used  which  will  give  results 
quickly.  This  can  be  done  with  a  pipette  cut  off  at  one  end,  or 
with  a  straight  glass  tube  contracted  at  one  end.  These  instruments 
are  not  always  arranged  so  the  temperature  of  the  glue  within  them 
can  be  controlled,  and  for  a  number  of  other  reasons  they  are  not 
entirely  accurate.  Better  control  of  temperature  and  greater  accu- 
racy can  be  had  with  the  Engler  viscosimeter.  This  is  more  com- 
plicated and  more  expensive  than  the  glass  tubes  and  also  slower  to 
operate,  but  it  has  the  advantage,  in  addition  to  greater  accuracy, 
of  being  an  instrument  which  is  in  general  use  for  testing  many  kinds 


.46  INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD. 

of  materials.  The  values  obtained  by  its  use  are  readily  understood 
by  laboratory  men  and  can  be  readily  checked.  The  instrument 
can  be  purchased  standardized  and  ready  for  use. 

Jelly  strength. — The  term  " jelly  strength"  refers  to  the  firmness  or 
strength  of  the  jelly  formed  by  a  glue  solution  of  specified  strength 
upon  cooling.  Strong  glues  usually  have  high  jelly  strength.  There 
is  no  standard  instrument  for  determining  jelly  strength  and  no 
standard  unit  for  expressing  it.  In  some  laboratories  the  pressure 
required  to  break  the  surface  of  the  jelly  is  measured.  In  others  the 
depth  to  which  a  weight  of  special  shape  will  sink  is  observed.  Some- 
times the  jelly  is  cast  in  a  conical  shape,  and  the  weight  required  to 
press  the  point  of  the  cone  a  certain  distance  is  taken.  More  com- 
mon, however,  is  the  finger  test,  in  which  the  relative  strength  of 
two  or  more  jellies  is  compared  by  pressing  the  jelly  with  the  fingers. 
In  making  this  test  with  any  apparatus  it  is  important  that  the  con- 
ditions be  very  carefully  controlled  in  order  that  comparative  results 
may  be  obtained.  The  temperature  of  the  jelly  when  tested  is  par- 
ticularly important,  as  the  relative  strength  of  a  number  of  jellies  is 
not  the  same  at  different  temperatures.  In  other  words,  the  jelly 
strength  of  the  different  glues  is  not  affected  to  the  same  extent  by 
changes  in  temperature.  The  ideal  condition  is  to  cool  and  test  the 
jellies  in  a  room  constantly  maintained  at  the  proper  temperature. 
This  is  seldom  practicable,  however,  and  the  jellies  must  be  cooled  in 
a  refrigerator  and  tested  in  a  warmer  room.  When  this  is  done  it 
is  important  that  the  test  be  made  as  quickly  as  possible  after  remov- 
ing the  jelly  from  the  refrigerator,  so  that  the  temperature  will  be 
practically  the  same  as  it  was  in  the  refrigerator.  The  strength  of 
the  glue  solution  must  always  be  the  same,  once  a  standard  is  adopted. 
For  high-strength  glues  weaker  solutions  can  be  used  than  for  low- 
strength  glues. 

Odor. — The  odor  of  a  glue  is  determined  by  smelling  a  hot  solution, 
and  gives  some  indication  of  its  source  or  its  condition.  Glue  which 
has  an  offensive  odor  is  not  considered  of  the  highest  grade.  The 
bad  odor  may  be  due  to  .the  fact  that  partly  decomposed  stock  was 
used,  or  that  the  glue  itself  is  decaying.  For  high-grade  work  it  is 
usually  specified  that  the  glue  be  sweet;  that  is,  it  must  not  have  an 
offensive  odor.  The  odor  of  different  glues  varies  considerably,  and 
it  is  difficult  or  impossible  to  express  the  different  "shades."  It  is 
usually  not  difficult,  however,  to  determine  whether  or  not  the  odor 
is  clean,  or,  as  it  is  commonly  called,  sweet.  The  temperature  and 
strength  of  solution  are  not  usually  specified. 

Keeping  quality. — The  keeping  quality  of  a  glue  is  determined  by 
allowing  the  jelly  left  from  the  jelly-strength  test  to  stand  in  the 
laboratory  at  room  temperature  for  a  number  of  days.  The  odor 
and  condition  of  the  glue  is  noted  at  intervals.  Glues  with  good 


INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD.  47 

keeping  qualities  will  stand  several  days  without  developing  an 
offensive  odor  or  showing  any  appearance  of  decomposition. 

Grease  tests. — For  joint  work  a  small  amount  of  grease  in  glue  is 
not  a  serious  objection.  Too  much  grease,  however,  is  objection- 
able, as  grease  has  no  adhesive  properties.  The  grease  can  be  deter- 
mined by  chemical  means,  if  desired,  but  this  is  not  necessary  unless 
the  exact  amount  of  grease  must  be  determined.  The  common 
method  of  testing  for  grease  is  to  mix  a  little  dye  with  the  glue  solu- 
tion and  paint  it  upon  a  piece  of  unsized  white  paper.  If  grease  is 
present,  the  painted  streak  will  have  a  mottled  or  spotted  appearance. 
If  there  is  no  grease  present,  the  streak  will  have  a  uniform  appear- 
ance 

Foaming. — Glue  which  foams  badly  is  objectionable,  because  air 
bubbles  are  apt  to  get  into  the  joint  and  thus  reduce  the  area  over 
which  the  glue  is  in  contact  with  both  faces.  Foamy  glue  is  espe- 
cially undesirable  for  use  in  gluing  machines,  as  in  them  the  glue  is 
agitated  much  more  than  when  it  is  used  by  hand,  and  the  danger 
of  incorporating  air  bubbles  is  greater.  The  amount  of  foam  is 
tested  by  beating  the  glue  solution  for  a  specified  time  with  an  egg 
beater  or  similar  instrument,  and  then  noting  the  height  to  which 
the  foam  rises  and  the  quickness  with  which  it  subsides.  Different 
laboratories  do  not  make  the  test  in  exactly  the  same  way,  but  in 
any  laboratory,  after  a  method  is  once  adopted,  it  should  be  strictly 
adhered  to  thereafter.  It  is  common  to  determine  the  foam  on  the 
solution  used  in  the  viscosity  test. 

Acid  test. — By  its  reaction  to  litmus  a  glue  shows  whether  it  is 
acid,  alkaline,  or  neutral.  The  test  is  made  by  dipping  strips  of  red 
and  blue  litmus  paper  in  the  glue  solution  remaining  after  the  vis- 
cosity test  or  some  other  test  and  noting  the  color  change.  An  acid 
glue  turns  blue  litmus  red,  an  alkaline  glue  turns  red  litmus  blue,  and 
a  neutral  glue  will  not  change  the  color  of  either  red  or  blue  litmus. 
A  glue  containing  a  slight  amount  of  acid  is  slightly  preferable  to 
one  which  is  neutral  or  alkaline,  because  it  is  not  quite  so  favorable 
a  medium  for  the  growth  of  the  organisms  which  cause  the  decay  of 
glue. 

Comparative  results  of  tests  on  glues. — From  the  above  description 
of  the  various  glue  tests,  it  is  apparent  that,  for  the  most  part,  they 
give  comparative  rather  than  absolute  results.  It  is  rather  difficult 
to  compare  the  results  of  tests  made  by  one  laboratory  with  those  of 
another,  as  the  strength  of  solution,  temperature,  and  manipulation 
are  often  different.  For  this  reason,  the  most  satisfactory  method 
of  purchasing  glues  is  to  specify  that  they  must  be  equal  to  a  stand- 
ard sample  which  is  furnished  the  bidder  to  test  in  any  way  he  sees 
fit.  The  bidder  should  also  be  informed  as  to  the  methods  the  pur- 
chaser proposes  to  use  in  testing  a  glue  submitted  to  him  as  equal  to 
the  standard  sample. 


48  INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD. 

PRECAUTIONS    IN    USING   GLUE. 

Preparation  of  glue. — In  using  hide  glue  there  are  a  number  of 
precautions  that  must  be  observed  to  obtain  satisfactory  results.  If 
improperly  used,  a  very  high-grade  glue  may  give  poor  joints.  It  is 
important,  first,  to  find  out  the  right  proportion  of  glue  and  water 
to  get  the  best  results.  This  is  largely  a  matter  of  experience,  but 
it  can  also  be  determined  by  strength  tests.  When  the  right  pro- 
portions have  been  decided  upon,  they  should  be  strictly  adhered 
to  thereafter,  and  the  glue  and  water  should  be  weighed  out  when 
making  up  a  new  batch  of  glue,  rather  than  measured  or  guessed  at. 
Clean  cold  water  should  be  put  on  the  glue,  which  should  be  allowed 
to  stand  in  a  cool  place  until  it  is  thoroughly  water  soaked  and  soft- 
ened. This  may  take  only  an  hour,  or  it  may  take  all  night,  depend- 
ing upon  the  size  of  the  glue  particles.  When  the  glue  is  soft,  it 
should  be  melted  over  a  water  bath,  and  the  temperature  not  allowed 
to  go  higher  than  about  150°  F.  High  temperatures  and  long- 
continued  heating  reduce  the  strength  of  the  glue  solution  and  are 
to  be  avoided.  The  glue  pot  should  be  kept  covered  as  much  as 
possible  in  order  to  prevent  the  formation  of  a  skin  or  scum  over  the 
surface  of  the  glue 

"Working  temperature.— The  room  in  which  the  glue  is  used  should 
be  as  warm  as  possible  without  causing  too  much  discomfort  to  the 
workmen,  and  it  should  be  free  from  drafts.  In  a  cold,  drafty  room 
the  glue  cools  too  quickly,  and  is  apt  to  set  before  the  joint  has  been 
put  into  the  clamps.  This  results  in  weak  joints.  It  is  also  con- 
sidered good  practice  to  warm  the  wood  before  applying  the  glue. 
Wood  should  never  be  glued  when  it  is  cold,  and  of  course  only 
thoroughly  seasoned  wood  should  be  used.  Since  high-strength 
animal  glues  set  so  quickly  on  cooling,  they  should  be  applied  and  the 
joints  clamped  as  quickly  as  consistent  with  good  workmanship. 

Clamping  of  glued  joints. — In  clamping  glued  joints  the  pressure 
should  be  evenly  distributed  over  the  joint,  so  that  the  faces  will  be 
in  contact  at  all  points.  The  amount  of  pressure  which  will  give 
the  best  results  is  a  question  which  has  never  been  definitely  settled. 
One  experimenter  found  that  a  pressure  of  about  30  pounds  per 
square  inch  gave  better  results  on  end  joints  than  higher  or  lower 
pressures.  Apparently  no  tests  have  yet  been  made  to  show  the 
best  pressure  to  use  on  edge  or  flat  grain  joints.  In  gluing  veneers 
it  is  necessary  to  use  high  pressure  in  order  to  flatten  out  the  irreg- 
ularities of  the  laminations.  Pressures  as  high  as  150  or  200  pounds 
per  square  inch  are  sometimes  used. 

Glue-room  sanitation. — Strict  cleanliness  of  glue  pots  and  appa- 
ratus and  of  the  floors  and  tables  of  the  glue  room  should  be  observed. 
Old  glue  soon  becomes  foul  and  affords  a  breeding  place  for  the 


FIG.  21.— Section  of  western  yellow  pine  log  showing:  Radial  surface,  R;  tan- 
gential surface,  T;  heartwopd,  H;  sapwood,  S;  pith,  P.  The  annual  rings  are 
the  concentric  layers  widest  near  the  pith  and  becoming  narrower  toward  the 
bark. 


INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD.  49 

bacteria  which  decompose  glue.  The  fresh  glue  is  therefore  in  con- 
stant danger  of  becoming  contaminated.  Glue  pots  should  be 
washed  after  every  day's  run  in  hot  weather,  and  two  or  three  times 
a  week  in  cooler  weather.  Only  enough  glue  for  a  day's  run  should 
be  mixed  at  a  time,  so  that  mixed  glue  will  not  have  to  be  held  over 
from  one  day  to  another.  If  these  sanitary  precautions  are  not 
observed,  poor  joints  are  apt  to  be  the  result. 

REFERENCES. 

The  glue  user  or  inspector  should  read  some  or  all  of  the  following 
publications : 

Glues  and  Gelatines.    R.  L.  Fernbach,  200  pages,  by  D.  Van  Nostrand,  New  York. 

Discusses  the  manufacture,  classification,  testing,  and  analysis  of  glues  and  gelatines 

and  gives  information  on  substitutes. 
Glue  and  Glue  Testing.    Samuel  Eideal,  140  pages,  published  by  Scott,  Greenwood  & 

Son,  London.     Discusses  the  manufacture,  testing,  and  use  of  glues,  especially  the 

chemical  side. 
The  Glue  Book.    J.  A.  Taggart,  Toledo,  Ohio,  85  pages.    Discusses  "how  to  select, 

prepare,  and  use  glue." 
Specifications  and  Tests  of  Glue.    Linder  and  Frost,  Proceedings  American  Society 

for  Testing  Materials,  1914,  Part  2,  pages  509  to  519.    Gives  the  results  of  tests  of 

cabinet  glue. 

A  Study  of  Various  Tests  upon  Glue.    A.  M.  Gill,  Journal  of  Industrial  and  Engineer- 
ing Chemistry,  Vol.  7  (1915),  pages  102  to  106.    Gives  the  results  of  tests  made  at 

Massachusetts  Institute  of  Technology. 
Glue  for  Use  on  Airplanes.     P.  A.  Houseman,  Journal  of  Industrial  and  Engineering 

Chemistry,  Vol.  9  (1917),  pages  359  to  360.     Republished  in  Aviation,  July  1,  1917, 

pages  494  to  495;  and  in  Aerial  Age  Weekly,  June  18,  1917,  page  462. 
The  Grading  and  Use  of  Glues  and  Gelatine.    Jerome  Alexander,  Journal  of  Society 

of  Chemical  Industry,  Feb.  28,  1906. 

THE  STRUCTURE  AND  IDENTIFICATION  OF  WOOD. 

A  knowledge  of  the  structure  of  wood  is  essential  in  the  identifica- 
tion and  proper  selection  of  lumber  used  in  airplanes.  Color,  odor, 
taste,  and  weight  are  also  very  helpful  in  identification,  but  these 
are  usually  more  variable  than  the  structure,  and  can  not  be  described 
as  accurately.  Furthermore,  these  properties  change  with  different 
treatments  of  the  wood,  while  the  structure  remains  the  same,  until 
disintegrated. 

HEARTWOOD   AND    SAPWOOD. 

Three  regions  are  usually  discernible  in  the  end  surface  of  a  log; 
an  inner  dark  core  called  the  heartwood,  the  bark,  and  between  the 
two  a  lighter  colored  portion  called  the  sapwood.  (See  section  of 
western  yellow  pine  log,  fig.  21.) 

In  some  woods  there  is  little  difference  in  the  color  between  the 
sapwood  and  the  heartwood,  there  being  no  sharp  line  of  demarcation 


50  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

between  the  two.  The  spruces,  hemlock,  Port  Orford  cedar,  bass- 
wood,  cottonwood,  beech  (white  heart),  and  hackberry  are  examples 
of  this  class. 

Sap  stain,  or  blue  stain,  discolors  the  sap  wood,  especially  in  the 
pines,  red  gum  and  hackberry,  but  does  not  weaken  it. 

The  width  of  the  sapwood  varies  with  the  age  and  vigor  of  the 
tree,  the  distance  from  the  stump,  and  the  species.  The  inner  portion 
of  the  sapwood  of  a  living  tree  gradually  changes  to  heartwood  from 
year  to  year  and  in  so  doing  usually  becomes  darker  and  more  resistant 
to  decay,  but  does  not  become  stronger. 

Certain  species  normally  have  very  narrow  sapwood,  and  others 
very  wide  sapwood,  and  this  feature  is  often  useful  in  identifying 
woods. 

Species  with  very  narrow  sapwood,  usually  less  than  1  inch  wide: 
Arbor- vitse,  western  red  cedar,  black  ash,  slippery  elm. 

Species  with  very  wide  sapwood,  usually  several  inches:  Maple, 
birch,  white. ash,  green  ash,  hackberry,  some  hard  pines. 

ANNUAL   RINGS. 

Annual  rings  are  the  well-defined  concentric  layers  of  wood  seen 
on  cross-sections  of  timbers  grown  in  temperate  climates  (see  fig.  21). 
Woods  grown  in  the  Tropics  usually  show  no  well-defined  annual 
rings  because  growth  continues,  more  or  less,  during  the  entire  year. 

Wood  can  be  cut  in  three  distinct  planes  with  respect  to  the  annual 
layers  of  growth:  Crosswise,  exposing  the  transverse  surface  or  end 
grain;  lengthwise  through  the  center  or  pith,  exposing  the  radial, 
or  "  quartered,"  surface  or  edge  grain,  sometimes  called  "  vertical 
grain";  and  lengthwise,  not  through  the  center,  exposing  the  tan- 
gential, or  " bastard,"  surface  or  flat  grain  (see  fig.  21). 

SPRINGWOOD  .AND    SUMMER  WOOD. 

Springwood  is  the  more  porous  and  softer  inner  part  of  each  annual 
ring  formed  during  the  early  part  of  the  growing  season  (see  PI.  II). 

Summerwood  is  the  less  porous  and  denser  outer  part  of  each  annual 
ring  formed  during  the  latter  part  of  the  growing -seas  on.  The  spring- 
wood  and  summerwood  may  each  be  sharply  defined  or  the  transition 
may  be  gradual.  In  some  woods,  for  example,  maple,  birch,  bass- 
wood,  cottonwood,  and  red  gum,  the  annual  rings  are  of  such  uniform 
structure  that  the  distinction  between  springwood  and  summerwood 
is  not  clear. 

THE    STRUCTURE    OF    HARDWOODS. 

Our  commercial  woods  can  be  divided  into  two  distinct  classes, 
the  hardwoods,  or  woods  from  broad-leaved  trees,  and  the  conifers, 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD.  51 

or  woods  from  trees  with  needle  or  scale-like  leaves.  The  conifers 
are  also  known  as  "softwoods,"  although  some  species,  as  the  hard 
pine,  tamarack,  etc.,  are  harder  than  basswood  and  cotton  wood, 
which  belong  to  the  hardwood  class. 

All  woods  are  composed  of  cells  joined  together  by  their  walls 
similar  to  the  cells  of  a  honeycomb,  but  much  smaller,  more  irregular 
in  size,  and  longer  in  proportion  to  their  width. 

Pores  or  vessels  are  larger  cells  found  only  in  the  hardwoods,  in 
which  they  are  scattered  among  the  smaller  cells  (see  PL  II,  oppo- 
site page  50) .  The  pores  are  visible  without  a  hand  lens  on  a  smoothly 
cut  end  surface  of  oak,  ash,  elm,  chestnut,  hackberry,  hickory,  black 
walnut,  and  mahogany.  In  other  hardwoods  the  pores  can  be  seen  dis- 
tinctly only  with  a  magnifying  glass,  but  they  are  always  larger  than 
the  surrounding  cells.  The  size  and  arrangement  of  the  pores  differ  in 
the  various  species  and  are  valuable  aids  in  identification.  (See 
Key  and  illustrations.)  On  account  of  the  presence  of  pores,  hard- 
woods are  also  called  porous  woods. 

Tyloses  are  frothlike  growths  in  the  pores  of  the  heartwood  and, 
occasionally,  of  the  inner  sap  wood,  often  closing  the  pores  com- 
pletely (see  Pi.  II).  The  absence,  or  presence,  of  tyloses  helps  to 
distinguish  certain  woods,  especially  the  white  oaks  (present)  from 
the  red  oaks  (absent). 

Some  woods  have  light-colored  tissue  (composed  of  very  small 
parenchyma  cells)  extending  around  the  pores  or  between  them  in 
fine  lines.  The  arrangement  of  these  light-colored  lines  helps  to 
identify  ash  and  hickory. 

Medullary  rays,  hereafter  referred  to  as  rays,  are  narrow  strips  of 
cells  extending  radially  in  the  wood  at  right  angles  to  the  grain. 
On  the  end  surface  they  appear  as  lines  crossing  the  annual  rings 
(see  PL  II).  They  are  largest  in  oak,  in  which  they  may  be  up  to  4 
inches  wide  with  the  grain,  forming  the  beautiful  " silver  grain"  of 
quarter-sawed  oak.  In  beech,  and  to  a  less  extent  in  maple,  they 
form  conspicuous  " flakes"  on  the  radial  surface.  In  other  woods 
they  are  less  distinct  on  the  radial  surface,  and  often  are  not  visible 
on  the  end  surface  without  a  lens. 

Wood  fibers  are  narrow,  thick-walled  fibrous  cells  scattered  among 
the  pores.  They  are  too  small  to  be  recognized  individually  with  a 
hand  lens,  collectively  they  form  the  darker  and  denser  portions 
which  give  most  of  the  weight  and  strength  to  wood  (see  PL  II). 

THE    STRUCTURE    OF   CONIFERS. 

In  conifers  the  bulk  of  the  wood  is   composed  of  fibrous  cells 
(tracheids)  which  serve  the  combined  purpose  of  the  pores  and  wood 
fibers  of  hardwoods.     They  are  of  almost  uniform  width  tangentially 
84727—19 5 


52  INFORMATION   FOE  INSPECTORS   OF   AIRPLANE   WOOD. 

and  are  arranged  in  definite  radial  rows  (see  PL  II  and  other  illustra- 
tions of  cross  sections  of  conifers).  On  account  of  the  absence  of 
pores,  the  conifers  are  also  called  nonporous  woods,  although  "  po- 
rous" in  the  sense  of  containing  empty  spaces  or  absorbing  liquids 
applies  to  both  conifers  and  hardwoods.  On  a  smoothly  cut  end 
surface  the  fibrous  cells  can  be  seen  with  a  hand  lens,  resembling  in 
their  regularity  the  cells  of  a  honeycomb.  In  the  outer  part  of  each 
annual  ring  they  are  flattened  radially  and  thicker-walled,  produc- 
ing a  denser  band  of  summerwood. 

Rays  are  also  present  in  conifers  but  they  are  always  very  small, 
and  on  the  end  surface  are  invisible  without  a  lens. 

Kesin  ducts  are  passages  extending  vertically  between  the  fibrous 
cells  and  radially  within  certain  medullary  rays.  They  serve  for 
the  conduction  of  resin  and  are  present  normally  only  in  the  pines, 
spruces,  larches,  or  tamaracks,  and  Douglas  fir  (see  illustrations  of 
cross  sections  of  these  woods  and  PL  II). 

In  the  pines  the  resin  ducts  are  plainly  visible  with  a  lens  and 
occasionally,  on  a  smoothly  cut  end  surface,  barely  visible  without  a 
lens.  On  longitudinal  surfaces  they  are  often  visible  as  character- 
istic brownish  lines.  In  the  spruces,  larches,  and  Douglas  fir  they 
are  smaller,  less  numerous,  and  in  the  cross  section  often  in  short 
tangential  rows,  appearing  as  whitish  specks  in  the  summerwood. 
On  longitudinal  surfaces  of  spruce  and  Douglas  fir  the  resin  ducts 
are  less  distinct  than  in  the  pines  but  can  usually  be  found  on  careful 
examination.  Since  the  resin  ducts  extend  in  the  same  direction  as 
the  fibers,  the  direction  of  the  grain  can  be  determined  by  them. 

Exudations  of  resin  at  the  ends  and  pitch  pockets  are  common  in 
woods  containing  resin  ducts,  and  are  not  found  in  the  cedars,  cy- 
press, redwood;  hemlock,  and  balsam  firs,  which  have  no  resin  ducts. 
Oily  exudations  have  been  noted  on  the  ends  of  Port  Orford  cedar 
stored  in  a  warm  place.  The  absence  of  exudations  of  resin,  how- 
ever, does  not  mean  the  absence  of  resin  ducts.  Resin  will  not 
exude,  as  a  rule,  on  cuts  made  after  the  wood  is  seasoned;  but  warm- 
ing pieces  of  pine,  Douglas  fir,  larch,  and  spruce  in  an  oven  will 
usually  cause  enough  resin  to  exude  from  the  ends  to  form  specks, 
thereby  indicating  the  presence  of  resin  ducts.  This  is  especially 
true  of  pine  and  Douglas  fir  and  to  a  less  extent  of  spruce  and  larch. 

PHYSICAL   PROPERTIES    USEFUL    IN    IDENTIFICATION. 

Color. — Since  wood  turns  darker  on  exposure  to  air  and  since  all 
sapwood  is  light-colored,  observations  as  to  color  should  be  made 
on  freshly  cut  longitudinal  surfaces  of  the  heartwood.  It  is  prac- 
tically impossible  to  describe  satisfactorily  the  different  colors  of 


INFORMATION    FOR  INSPECTORS   OF   AIRPLANE   WOOD.  53 

wood,  therefore  comparisons  should  be  made  with  known  samples 
whenever  possible. 

Odor  and  taste. — Many  species  give  off  a  characteristic  odor  when 
worked.  Therefore,  to  get  the  odor,  wood  should  be  whittled,  or 
better  yet,  sawed  and  the  sawdust  held  to  the  nostrils.  The  taste 
of  wood  is  usually  similar  to  the  odor,  although  in  the  cedars  the 
taste  is  more  spicy  or  bitter.  Cypress  has  somewhat  rancid  odor 
but  is  practically  tasteless.  Odor  and  taste  a're  more  pronounced 
heartwood  than  in  the  sap  wood. 

Weight. — When  wood  is  dry,  its  weight  aids  in  identification, 
although  a  species  is  often  highly  variable  in  weight.  For  instance, 
the  heavier  grades  of  mahogany  may  weigh  twice  as  much  as  the 
lighter  grades,  which  often  are  sold  under  the  name  of  "baywood." 

No  definite  weight  can  properly  be  assigned  to  each  species,  and 
the  following  comparative  classification  is  here  used  because  it  is 
more  descriptive  than  the  average  weight  expressed  in  pounds  per 
cubic  foot : 

Very  light,  up  to  19  pounds  oven-dry  weight  per  cubic  foot  original  green  volume. 

Light,  19  to  22  pounds,  oven-dry  weight  per  cubic  foot  original  green  volume. 

Moderately  light,  22  to  26  pounds,  oven-dry  weight  per  cubic  foot  original  green 
volume. 

Moderately  heavy,  26  to  31  pounds  oven-dry  weight  per  cubic  foot  original  green 
volume. 

Heavy,  31  to  37  pounds  oven-dry  weight  per  cubic  foot  original  green  volume. 

Very  heavy,  37  to  45  pounds  oven-dry  weight  per  cubic  foot  original  green  volume. 

Very,  very  heavy,  45  or  more  pounds  per  cubic  foot  original  green  volume. 

GRAIN   AND   TEXTURE. 

In  this  publication  " grain"  is  used  with  respect  to  the  direction 
of  the  fibers,  as  straight,  diagonal,  cross,  and  spiral  or  twisted  grain, 
and  with  respect  to  the  plane  in  which  lumber  is  cut,  as  end  grain, 
flat  grain,  and  edge  (also  known  as  "vertical"  and  "comb")  grain. 
"Grain"  has  often  been  used  to  indicate  the  width  of  the  rings, 
especially  when  there  is  a  decided  difference  between  spring  wood  and 
summer  wood  so  as  to  make  the  rings  conspicuous  as  in  Douglas  fir 
and  yellow  pine.  However,  to  avoid  confusion,  it  is  better  to  ex- 
press the  width  of  the  rings  as  narrow,  medium,  or  wide,  or  as  a 
certain  number  of  rings  per  inch  of  radius. 

"Texture"  is  here  used  to  indicate  the  relative  size  of  the  pores  of 
hardwoods  or  fibrous  cells  of  tracheids;  for  instance,  coarse  texture 
for  woods  with  large  pores,  such  as  oak,  ash,  chestnut,  etc.,  and  fine 
texture  for  hardwoods  with  comparatively  small  pores,  such  as  maple, 
birch,  red  gum,  etc.,  and  all  coniferous  woods,  although  some  conifers 
have  slightly  larger  cells  than  others.  In  wood  finishing,  lumber 
with  large  pores  has  often  been  called  "open-grained"  or  "coarse- 


54  INFORMATION   FOR  INSPECTORS  OF  AIRPLANE   WOOD. 

grained"  and  with  small  pores  "close-grained"  or  "fine-grained"; 
but  the  use  of  the  word  "grain"  with  so  many  different  meanings 
is  confusing  and  should  be  avoided  if  possible.  " Texture"  is  also 
used  to  designate  the  even  or  uneven  structure  of  the  annual  rings; 
wood  with  decided  contrast  between  springwood  and  summerwood  is 
said  to  have  an  uneven  texture,  as  ash,  oak,  yellow  pine,  and  Douglas 
fir;  and  wood  with  little  contrast  between  springwood  and  summer- 
wood  is  said  to  have  an  even  texture,  as  white  pine,  Port  Orford 
cedar,  yellow  poplar,  and  birch. 

PROCEDURE    IN    IDENTIFYING    WOOD. 

If  the  color,  odor,  or  general  appearance  is  not  sufficiently  distinct 
to  identify  a  sample  of  wood,  the  more  detailed  structure  must  be 
taken  into  consideration.  The  structure  and  other  physical  prop- 
erties of  the  various  species  that  the  inspector  is  likely  to  meet  are 
described  in  the  following  pages,  and  a  key  has  been  prepared  which 
will  aid  in  their  identification.  The  illustrations  of  the  woods  are 
photographs  of  thin  cross-sections  magnified  15  diameters.  They 
will  prove  helpful  in  studying  the  structure  of  each  species  or  group 
of  species. 

The  characteristic  structure  is  usually  seen  to  best  advantage  on  a 
smoothly  and  freshly  cut  end  surface  across  rings  of  average  width. 
The  area  examined  need  not  be  large,  but  it  is  advisable  to  make  ob- 
servations at  several  places.  Note  first  if  pores  are  present.  If 
pores  can  not  be  seen  with  the  unaided  eye,  use  a  hand  lens.  A  lens 
magnifying  about  15  diameters  is  preferable  for  this  work.  The 
lens  should  be  held  close  to  the  eye  and  then  the  object  brought 
within  focus,  care  being  taken  not  to  shut  out  the  light  too  much. 
If  pores  are  present,  note  whether  the  wood  is  ring  porous  or  diffuse 
porous,  etc.,  as  outlined  in  the  key,  pages  55  to  58.  If  pores  are  not 
present,  try  to  classify  the  wood  according  to  the  subdivisions  under 
the  conifers. 

It  is  not  expected  that  the  key  can  be  used  successfully  without 
some  practice.  The  inspector  should  provide  himself  with  known 
samples  and  study  the  illustrations  of  cross-sections  in  this  book,  so 
as  to  become  familiar  with  what  is  meant  by  the  terms  used.  Sam- 
ples for  comparison  should  be  of  heartwood  of  the  tree  trunk,  show- 
ing average  width  of  rings  and  at  least  3  inches  from  the  center  or 
pith  of  the  tree. 


INFORMATION   FOR   INSPECTORS   OF   AIRPLANE   WOOD.  55 

KEY  FOR  THE  IDENTIFICATION  OF  WOOD  USEFUL  IN  THE  CONSTRUC- 
TION OF  AIRPLANES.1 

HARDWOODS. 

I.  Wood  with  pores.  The  pores  are  conspicuously  larger  than  the  surrounding  cells, 
although  in  some  species  they  are  not  visible  without  magnification.  Neither 
the  pores  nor  other  cells  are  in  continuous  radial  rows.  (For  "Wood  without 
pores"  see  II,  p.  57.) 

A.  Ring-porous;  that  is,  the  pores  at  the  beginning  of  each  annual  ring  are  com- 
paratively large,  forming  a  distinct  porous  ring,  and  decrease  in  size  more  or 
less  abruptly  toward  the  summerwood.  (For  Diffuse-porous  see  B,  p.  56.) 

1.  Summerwood  figured  with  wavy  or  branched  radial  bands  visible  without  a 

lens  on  a  smoothly  cut  end  surface. 
A  A.  Many  rays  very  broad  and  conspicuous.     Wood  heavy  to  very  heavy. 

The  OAKS. 

(at)  Pores  in  the  summerwood  very  small  and  so  numerous  as  to  be  exceed- 
ingly difficult  to  count  under  a  lens;  pores  in  the  springwood  of  the 
heartwood  densely  plugged  with  ty loses,  except  in  chestnut  oak,  in 
which  they  are  more  open.  Heartwood  brown,  usually  without  reddish 

tinge. 

The  WHITE-OAK  GROUP,  p.  59. 

(bO  Pores  in  the  summerwood  larger,  distinctly  visible  with  (sometimes 
without)  a  hand  lens  and  not  so  numerous  but  that  they  can  readily 
be  counted  under  a  lens;  pores  in  the  springwood  mostly  open;  ty  loses 
not  abundant.  Heartwood  brown,  with  reddish  tinge,  especially  in 
the  vicinity  of  knots. 

The  RED  OAK  GROUP,  p.  60. 
BB.  All  rays  very  fine  and  inconspicuous.     Color,   grayish-brown.    Wood 

moderately  light. 

CHESTNUT,  p.  60. 

2.  Summerwood  figured  with  long  or  short  wavy  tangential  bands  visible  without 

a  lens  on  a  smoothly  cut  end  surface. 

AA.  Careful  examination  with  a  hand  lens  shows  that  the  pores  of  the  summer- 
wood  are  very  numerous  and  joined  so  as  to  form  more  or  less  wavy 
tangential  bands, 
(a^  Heartwood  light-  to  deep-reddish  brown.    Rays  not  distinct  without  a 

lens. 
(a2)  'Large  pores  in  the  springwood  usually  in  one  row,  except  in  very  wide 

rings. 

(a3)  Rows  of  pores  in  the  springwood  conspicuous  because  they  are  large 
enough  to  be  plainly  visible  without 'a  lens;  they  are  mostly 
open,  containing  only  a  few  tyloses;  and  they  are  fairly  close 
together.  Wood  moderately  heavy;  fairly  easy  to  cut. 

WHITE  ELM,  p.  61. 

(b3)  Rows  of  pores  in  the  springwood  inconspicuous  because  they  are 
small,  being  barely  visible  without  a  lens;  they  are  mostly  closed 
with  tyloses,  especially  in  the  heartwood;  and  they  are  often 
somewhat  separated.  Wood  heavy  and  difficult  to  cut. 

CORK  OR  ROCK  ELM,  p.  61. 

(b2)  Large  pores  in  springwood  in  several  rows;  mostly  open,  containing 
few  tyloses.    Wood  moderately  heavy. 

SLIPPERY  ELM,  p.  61. 

1  Unless  otherwise  directed,  all  observations  as  to  structures  should  be  made  on  the  end  surface  of  rings 
of  average  width  cut  smoothly  with  a  very  sharp  knife;  and  all  observations  as  to  color  should  be  made 
on  freshly  cut  longitudinal  surfaces  of  the  heartwood. 


56  INFOKMATION   FOR   INSPECTOKS   OF   AIRPLANE   WOOD. 

(bt)  Heartwood  yellowish-  to  greenish-gray.  Kays  distinct  without  a  lens. 
Pores  in  the  springwood  mostly  open,  in  several  rows  except  in  occa- 
sional narrow  rings,  where  they  may  form  only  one  row.  Wood  moder- 
ately heavy. 

HACKBERRY,  p.  61. 

SUGARBERRY,  p.  61. 

BB.  Careful  examination  with  a  hand  lens  shows  the  pores  of  the  summerwood 
to  be  few  and  isolated  (or  occasionally  in  radial  rows  of  2  or  3),  but  sur- 
rounded by  light-colored  tissue  (parenchyma),  which  also  projects 
tangentially,  often  connecting  pores  widely  separated,  especially 
toward  the  outer  portion  of  the  annual  ring. 

(aa)  Projections  of  light-colored  tissue  from  the  pores  of  the  outer  summerwood 
comparatively  long  and  distinct.  Heartwood  grayish-brown,  occa- 
sionally with  reddish  tinge.  Wood  heavy  and  hard.  Sap  wood  wid6 
and  usually  present  in  wide  boards. 

WHITE  ASH,  p.  62. 
GREEN  ASH,  p.  62. 

(bi)  Projections  of  light-colored  tissue  from  the  pores  of  the  outer  summerwood 
short,  often  absent.  Heartwood  silvery-brown.  Wood  moderately 
heavy.  Sap  wood  usually  less  than  1  inch  wide  and  therefore  scarce  in 
lumber. 

BLACK  ASH,  p.  62. 

3.  Summerwood  not  figured  with  radial  or  tangential  bands  distinctly  visible 
without  a  lens.  Pores  in  the  summerwood  comparatively  few  and  isolated 
or  in  radial  rows  of  2  or  -3. 

AA.  Under  a  lens  numerous  fine  light-colored  tangential  lines  (parenchyma) 
are  plainly  visible.  Sap  wood  wide;  heartwood  reddish-brown.  Wood 
very  heavy  to  very,  very  heavy. 

HICKORIES,  p.  62. 

BB.  No  fine  lines  of  parenchyma  visible  except  occasional  short  projections  of 
parenchyma  from  the  outermost  pores  of  the  summerwood.  Sap  wood 
narrow;  heartwood  silvery-brown.  Wood  moderately  heavy. 

BLACK  ASH,  p.  62. 

B.  Diffuse-porous;  that  is,  the  pores  are  of  about  uniform  size  and  evenly  dis- 
tributed throughout  the  annual  ring;  or  if  they  are  slightly  larger  and  more 
numerous  in  the  springwood,  they  gradually  decrease  in  size  and  number 
toward  the  outer  edge  of  the  ring. 

1.  Pores  comparatively  large  and  conspicuous,  plainly  visible  without  a  lens 
AA.  Heartwood  chocolate-brown.     Wood  heavy  and  hard. 

BLACK  WALNUT,  p.  63. 
BB.  Heartwood  reddish-brown.     Many  pores  filled  with  dark  amber-colored 

gum.     Wood  moderately  heavy. 

(at)  Fine  light-colored  tangential  lines  indicating  annual  rings  and  varying 
from  y$  to  £  inch  apart,  plainly  visible  without  a  lens. 

(True)  MAHOGANY,  p.  63. 
(bt)  No  fine  light-colored  tangential  lines  present. 

AFRICAN  MAHOGANY,  p.  64. 

2.  Pores  not  plainly  visible  without  a  lens  (although  barely  visible  under  favor- 

able conditions  in  birch  and  cotton  wood). 

AA.  Numerous  rays,  broad  and  conspicuous,  fully  twice  as  wide  as  the  largest 
pores,  visible  on  the  radial  surface  as  "flakes"  about  J  inch  wide  with 
the  grain;  other  rays  very  fine.  Wood  heavy,  light  reddish-brown. 

BEECH,  p.  64. 


INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD.  57 

BB.  Rays  narrower,  ranging  from  very  distinct  and  same  size  as  pores  in 

maple  and  cherry,  to  barely  visible  with  a  lens,  as  in  cottonwood. 
(aj)  Heartwood  light  to  deep  reddish-brown. 

(a2)  Rays  very  distinct  without  a  lens  and  fully  as  wide  as  the  largest  pores. 
(a3)  Heartwood  deep  reddish-brown.    Pores  slightly  decreasing  in  size 
from  inner  to  outer  portion  of  each  annual  ring.    The  rays  con- 
spicuous on  the  radial  surface,  but  not  darker  than  the  surrounding 
wood.    Wood  moderately  heavy. 

BLACK  CHERRY,  p.  64. 

(b3)  Heartwood  very  light  reddish-brown.    Pores  of  uniform  size  through- 
out the  annual  ring.    The  rings  denned  by  thin  reddish-brown 
layers,  usually  conspicuous  on  the  longitudinal  surfaces.    The  rays 
conspicuous  on  the  radial  surface  as  reddish-brown  flakes  TJ  to  t^ 
'  inch  wide  with  the  grain;  on  the  end  surface  only  part  of  the  rays 

broad,  the  others  very  fine,  scarcely  visible  with  a  lens.    Wood 
heavy;  difficult  to  cut  across  the  grain. 

SUGAR  MAPLE,  p.  65. 
(b2)  Rays  not  very  distinct  without  a  lens  and  narrower  than  the  largest 

pores. 

(a3)  Pores  comparatively  large  under  a  lens,  and  barely  visible  without 
.  a  lens  under  conditions  of  good  light  and  a  smoothly  cut  end  sur- 
face, visible  on  a  smooth  longitudinal  surface  as  fine  grooves. 
Wood  heavy. 

SWEET  BIRCH,  p.  65. 
YELLOW  BIRCH,  p.  65. 

(b3)  Pores  very  small,  barely  visible  with  a  lens.    Heartwood  often 
figured  with  irregular  darker  streaks;  sap  wood  pinkish.    Wood 

moderately  heavy. 

RED  GUM,  p.  66. 

(bj)  Heartwood  yellowish  or  brownish  with  greenish  tinge.    Wood  moder- 
ately light. 

YELLOW  POPLAR,  p.  66. 
CUCUMBER,  p.  66. 

(Although  cucumber  averages  slightly  heavier  than  yellow  poplar,  there 
is  no  reliable  means  of  distinguishing  the  two  without  a  compound 
microscope.) 

(cj)  Heartwood  light-colored,  almost  white  or  grayish.    Wood  light  and  soft. 
(a2)  Color  creamy-white.    Rays  distinct  without  a  lens,   pores  barely 

visible  with  a  lens. 

BASSWOOD,  p.  66. 

(b2)  Color  grayish-white.    Rays  barely  visible  with  a  lens;  pores  very 
distinct  with  and  barely  visible  without  a  lens. 

COTTONWOOD,  p.  67. 

CONIFERS. 

II.  Wood  without  pores.  The  fibrous  cells  (tracheids)  very  small,  barely  visible  with 
a  lens;  practically  uniform  in  size  excepting  in  the  summerwood  where  they 
are  narrower  radially;  arranged  throughout  in  definite  radial  rows.  Rays  very 
fine. 

A.  Odor  aromatic,  resinous,  or  spicy;  taste  bitter  or  spicy.    Resin  ducts,  pitch 
pockets,  and  exudations  of  resin  on  end  surfaces  entirely  absent. 

The  CEDARS. 
1.  Heartwood  moderately  dark  to  dark  brown,  with  or  without  reddish  tinge. 

Odor  and  taste  like  cedar  shingles. 

AA.  Heartwood  plain  brown,  rarely  with  reddish  tinge.    Wood  very  light. 

ARBOR-VIT^:,  p.  67. 


58  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

BB.  Heartwood  reddish  brown.     Wood  light. 

(a^  Medullary  rays,  as  seen  under  a  hand  lens  on  freshly-split  radial  surface, 
orange-red  with  numerous  very  fine  amber-colored  specks  of  resin. 
Sap  wood  usually  over  1£  inches  wide. 

INCENSE  CEDAR,  p.  68. 

(b^  Medullary  rays,  as  seen  under  a  hand  lens  on  freshly-split  radial  surface, 
light  brown,  rarely  .containing  amber-colored  specks  of  resin.  Sap- 
wood  usually  less  than  1  inch  wide. 

WESTERN  RED  CEDAR,  p.  67. 

2.  Heartwood  very  pale  brown.     Wood  moderately  light  to  moderately  heavy. 
Odor  and  taste  not  like  cedar  shingles,  more  spicy. 

PORT  ORFORD  CEDAR,  p.  68. 

B.  Odor  neither  aromatic  nor  spicy,  but  maybe  resinous  or  "pitchy";  taste  neither 
bitter  nor  spicy.  Resin  ducts  present;  also  occasionally,  pitch  pockets  and 
exudations  of  resin  from  the  ends. 

1.  Resin  ducts  very  distinct  and  numerous,  appearing  in  the  end  surface  as 

minute  openings,  and  often  on  the  longitudinal  surface  as  brownish  lines. 

The  PINES. 

AA.  Summerwood  inconspicuous  and  not  appreciably  harder  than  the  spring- 
wood  in  cutting  across  the  grain.  Heartwood  light  reddish-  or  creamy- 
brown.  Wood  moderately  light. 

The  WHITE  PINE  GROUP:  EASTERN  WHITE  PINE,  p.  68; 
WESTERN  WHITE  PINE,  p.  68;  SUGAR  PINE,  p.  69. 

BB.  Summerwood  conspicuously  darker  and  harder  than  the  springwood 
appearing  as  a  glistening  layer  on  the  end  or  longitudinal  surfaces. 

The  HARD  PINES,  p.  69. 

(ax)  Wood  moderately  light.  Summerwood  narrow;  some  pieces  approxi- 
mating the  white  pines  in  appearance,  although  a  thin  layer  of  horny, 
glistening  summerwood  is  nearly  always  present. 

WESTERN  YELLOW  PINE,  p.  69. 

(bj)  Wood  moderately  heavy  to  heavy.     Summerwood  very  pronounced. 

NORWAY  PINE,  p.  69. 
SHORTLEAP  PINE,  p.  69. 
And  other  hard  pines. 

2.  Resin  ducts  indistinct,  not  numerous,  visible  as  whitish  specks  in  the  summer- 

wood;  pitch  pockets  and  exudations  of  resin  may  be  present. 

AA.  Heartwood  reddish.    Wood  moderately  heavy. 

DOUGLAS  FIR,  p.  70. 
BB.  Heartwood   pale  reddish-brown.    Wood   moderately   light. 

SITKA  SPRUCE,  p.  70. 
CO.  Heartwood  almost  white.    Wood  moderately  light. 

WHITE  SPRUCE,  p.  70. 
RED  SPRUCE,  p.  70. 

0.  Odor  somewhat  rancid;  wood  without  characteristic  taste.  Resin  ducts,  pitch 
pockets,  and  exudations  of  resin  absent.  Highly  variable  in  color  from  pale 
brown,  or  reddish-brown  to  almost  black.  Longitudinal  surfaces  feel  and 
appear  waxy.  Weight  variable  from  moderately  light  to  heavy. 

BALD  CYPRESS,  p.  71. 

D.  Odorless  and  tasteless.  Resin  ducts,  pitch  pockets,  and  exudations  of  resin 
absent.  Heartwood  moderately  light  to  deep  reddish-brown.  Wood  mod- 
erately light. 

REDWOOD,  p.  71. 


FIG.  22.— Post  oak.    Cross  section  magnified  15  diameters. 


FIG.  23.— Red  oak.    Cross  section  magnified  15  diameters. 


INFORMATION   FOE   INSPECTORS  OF  AIRPLANE  WOOD.  59 

DESCRIPTION  OF  WOODS  IN  KEY. 

The  scientific  names  are  those  used  by' the  Forest  Service  as  given  in  Bulletin  17,  "Check  List  of  Forest 
Trees  of  the  United  States." 

The  letters  after  the  names  refer  to  the  forest  regions  in  which  the  trees  grow,  as  indicated  on  the  map, 
PI.  I,  al thought  the  geographic  distribution  of  each  species  is  not  confined  exactly  to  the  limits  of  the  regions 
indicated. 

HARDWOODS. 

THE    WHITE   OAK   GROUP. 

The  following  commercial  species  belong  to  the  white  oak  group : 

WHITE  OAK  (Quercus  alba).    (A,  B,  D,  E.) 

BUR  OAK  (Quercus  macrocarpa).    (A,  B,  C,  D.) 

SWAMP  WHITE  OAK  (Quercus  platanoides).    (A,  B,  D.) 

POST  OAK  (Quercus  minor).    (B,  D.  E.) 

CHINQUAPIN  OAK  (Quercus  acuminata).    Yellow  oak.     (B,  D.) 

Cow  OAK  (Quercus  mwhauxii).    Basket  oak.     (E.) 

OVERCUP  OAK  (Quercus  lyrata).    (E.) 

CHESTNUT  OAK  (Quercus  prinus).    (B  and  adjacent  territory.) 

•  Most  of  the  white  oaks  are  so  much  alike  in  color  and  structure 
that  no  reliable  means  of  identifying  the  wood  of  each  species  has 
been  found.  Chestnut  oak  can  usually  be  distinguished  from  other 
white  oaks  by  the  more  open  pores  of  the  springwood  in  the  heart- 
wood. 

The  woods  of  the  white  oak  group  are  heavy  and  hard.  The  sap- 
wood  is  mostly  from  1  to  2  inches  wide.  The  heartwood  is  grayish 
brown,  usually  without  any  reddish  tinge.  The  dry  wood  is  without 
characteristic  odor  or  taste. 

The  annual  rings  are  made  very  distinct  by  the  large  pores  in  the 
springwood,  which  form  a  porous  ring  from  1  to  3  spores  wide.  In 
the  heartwood  these  pores  are  nearly  all  entirely  filled  with  tyloses, 
except  in  chestnut  oak,  in  which  they  are  more  open  but  not  so  much 
so  as  in  the  red  oaks. 

The  pores  of  the  summerwood  are  arranged  in  irregular,  branched 
or  wavy  radial  bands.  They  are  very  small  and  so  numerous  that 
they  are  difficult  to  count  even  under  a  good  hand  lens.  This  fea- 
ture is  an  absolutely  reliable  means  of  distinguishing  the  white  oak 
from  the  red  oaks,  the  summerwood  pores  in  the  latter  being  larger 
and  not  so  numerous.  (Compare  illustrations  of  white  oak  (PI.  II.) 
and  post  oak  (fig.  22),  with  illustration  of  red  oak  (fig.  23),  and  also 
samples  of  the  two  groups.) 

The  most  characteristic  feature  of  all  oak  woods,  including  the 
red  oak  and  live  oak  groups,  is  the  presence  of  certain  broad  medullary 
rays,  very  conspicuous  on  the  end  surface  and  appearing  on  the  radial 
surface  as  silvery  " patches"  from  one-half  an  inch  to  4  inches  in 
height  with  the  grain. 

Chestnut  resembles  plain-sawed  white  oak  but  is  lighter  and  has 
only  very  fine  rays. 


60  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

THE   RED   OAK   GROUP. 

The  following  commercial  species  belong  to  the  red  oak  group: 

RED  OAK  (Quercus  rubra).     (A,  B,  C,  D.) 

YELLOW  OAK  (Quercus  velutina).    Black  oak.     (A,  B,  D,  -E.) 

T-EXAN  OAK  (Quercus  texana).     (D,  E.) 

SPANISH  OAK  (Quercus  digitata).     (E.) 

PIN  OAK  (Quercus  palustris).     (B,  D.) 

SCARLET  OAK  (Quercus  coccinea).     (B,  D.) 

BLACK  JACK  (Quercus  marilandica).     (B,  D,  E.) 

WILLOW  OAK  (Quercus  phellos).     (E.) 

WATER  OAK  (Quercus  nigrd).     (E.) 

LAUREL  OAK  (Quercus  laurifolid).    (E.) 

The  wood  of  the  red  oaks  averages  about  as  heavy  as  that  of  the 
white  oaks.  The  sapwood  is  from  1  inch  to  3  inches  wide.  The 
heartwood  usually  has  a  reddish  tinge,  although  occasional  pieces 
resemble  white  oak  in  color.  The  dry  wood  is  without  characteristic 
odor  or  taste,  but  unseasoned  wood  has  a  sour  odor. 

The  annual  rings  average  wider  than  in  the  white  oaks,  and  as  a 
rule  are  more  distinct  because  the  springwood  consists  of  mostly 
open  pores  forming  a  porous  ring  from  2  to  4  (1  in  narrow  rings)  pores 
wide.  Black  jack  is  an  exception  in  that  the  springwood  pores  are 
mostly  closed  with  tyloses  as  in  the  white  oaks. 

The  pores  in  the  summer  wood  are  larger  but  less  numerous  than 
in  the  white  oak  group  and  can  easily  be  counted  under  a  hand  lense. 
(Compare  figs.  22  and  23.)  An  inspector  should  provide  himself  with 
a  half-inch  cube  of  heartwood  of  the  white  oak  group  and  one  of  the 
red  oak  group,  both  showing  rings  of  average  width  and  cut  smoothly 
across  the  ends.  These  cubes  may  be  tied  together,  thus  affording  a 
convenient  means  of  comparison. 

CHESTNUT  (Castanea  dentata).     (B,  D.) 

The  wood  of  chestnut  is  moderately  light  and  usually  straight- 
grained.  The  sapwood  is  generally  less  than  1  inch  wide.  The  heart- 
wood  is  grayish  brown,  without  characteristic  odor  but  with  a  slight 
astringent  taste  due  to  the  tannin  in  it. 

The  annual  rings  are  made  very  distinct  by  a  broad  band  of  porous 
springwood,  the  pores  being  plainly  visible  without  a  lens.  The  pores 
in  the  summerwood  are  very  numerous  and  arranged  in  irregular 
radial  bands,  similar  to  those  in  white  oak.  (See  fig.  24.) 

All  the  rays  are  very  fine  and  not  very  distinct  even  with  a  lens. 

The  heartwood  of  the  ashes,  especially  black  ash,  resembles  chest- 
nut somewhat;  but  the  pores  of  the  summerwood  in  the  ashes  are 
few  and  isolated;  and  never  crowded  and  in  radial  bands,  as  in  the 
chestnut.  Plain-sawed  oak  is  similar  in  appearance  to  chestnut  but 
is  much  heavier  and  contains  numerous  broad  and  conspicuous  rays. 


FIG.  24.— Chestnut.    Cross  section  magnified  15  diameters. 


FIG.  25.— White  elm.    Cross  section  magnified  15  diameters. 


FIG.  26.-- Cork  elm.    Cross  section  magnified  15  diameters 


FIG.  27. — Slippery  elm.    Cross  section  magnified  15  diameters. 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD.  61 

THE   ELMS. 

WHITE  ELM  ( Ulmus  americana).    (A,  B,  C,  D,  E.) 

SLIPPERY  ELM  (Ulmus  pubescens).    Red  elm.    (A,  B,  C,  D,  E.) 

CORK  ELM  ( Ulmus  racemosa).    Rock  elm.    (B.  D.) 

The  wood  of  the  white  and  slippery  elms  is  moderately  heavy  and 
easy  to  work;  that  of  the  cork  elm  is  heavier,  harder,  and  ranks 
higher  in  mechanical  properties. 

The  sapwood  varies  from  about  one-half  inch  wide  in  slippery  elm 
to  2  or  3  inches  wide  in  white  elm,  with  cork  elm  intermediate.  The 
heartwood  is  brownish,  usually  with  a  reddish  tinge.  The  wood  is 
considered  practically  tasteless  and  odorless,  but  slippery  elm  has  a 
slight  odor  resembling  that  of  the  bark,  which  is  familiar  to  many  on 
account  of  its  medicinal  properties. 

The  annual  rings  are  most  conspicuously  defined  in  slippery  elm 
and  least  in  cork  elm.  In  slippery  elm  the  springwood  consists  of 
several  rows  of  large  pores  as  a  rule,  but  in  white  elm  and  cork  elm 
only  one  row  of  large  pores  is  present  except  in  very  wide  annual 
rings.  In  cork  elm  the  springwood  pores  are  smaller  than  in  white 
elm  and  usually  filled  with  tyloses  in  the  heartwood,  so  as  to  make 
them  inconspicuous  on  a  cross-section.  (Compare  figs.  25,  26,  and 
27.)  This  difference  in  the  size  and  number  of  the  pores  of  the  spring- 
wood  is  probably  the  most  reliable  means  of  distinguishing  the 
species  of  elm.  The  pores  of  the  summerwood  of  all  the  elms  are 
very  numerous  and  joined  in  more  or  less  continuous  wavy  tangential 
lines  found  in  no  other  commercial  wood  except  hackberry.  Hack- 
berry,  however,  has  light  gray  heartwood  tinged  with  green ;  and  the 
rays  are  distinct  without  a  lens,  while  in  the  elms  they  are  not  visible 
to  the  unaided  eye. 
HACKBERRY  (Celtis  ocddentalis).  Bastard  elm.  (B,  D,  E,  F.) 

The  wood  of  hackberry  is  moderately  heavy  and  fairly  straight- 
grained.  The  sapwood  is  usually  over  3  inches  wide  in  saw  logs  and 
has  a  faint  greenish  yellow  color  or  is  blued  with  sap  stain.  The 
heartwood  is  light  gray  tinged  with  green.  It  is  without  character- 
istic odor  or  taste. 

The  annual  rings  are  clearly  defined  but  usually  irregular  in  width 
and  outline.  The  pores  of  the  springwood  are  comparatively  large, 
forming  a  zone  several  pores  (in  narrow  rings  one  pore)  wide.  In 
the  summerwood  the  pores  are  very  numerous  and  arranged  in  more 
or  less  continuous  wavy  tangential  lines  as  in  elm.  (See  fig.  28.) 

The  rays  in  hackberry  are  distinct  without  a  lens,  which,  together 
with  the  color,  distinguishes  it  from  the  elms. 

SUGARS ERRY  (Celtis  mississippienses)  (E  and  southern  part  of  D)  is  very  much  like 

hackberry  and  is  usually  sold  as  such. 

The  sapwood  of  these  species  might  be  mistaken  for  ash  sapwood, 
but  the  arrangement  of  the  pores  in  the  summerwood  is  entirely 
different*  (Compare  figs.  28  and  29.) 


62  INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD. 

THE    ASHES. 

WHITE  ASH  (Fraxinus  americana).  (A,  B,  C,  D,  E.) 
GREEN  ASH  (Fraxinus  lanceolata).  (A,  B,  C,  D,  E.) 
BLACK  ASH  (Fraxinus  nigra).  Brown  ash.  (A,  C,  and  northern  parts  of  B  and  D.) 

The  above  three  species  comprise  about  98  per  cent  of  all  the  ash 
cut.  The  white  ash  and  green  ash  are  very  much  alike  and  are  sold 
as  "white  ash"  or  "ash." 

The  sapwood  of  the  white  and  green  ashes  is  comparatively  wide 
and  white.  The  heartwood  is  grayish-brown,  occasionally  with  a 
reddish  tinge.  In  black  ash  the  sapwood  is  narrow,  usually  less 
than  1  inch  wide  and  the  heartwood  is  silvery  brown.  Black  ash 
averages  considerably  lighter  in  weight  than  the  other  two  species. 
Ash  wood,  especially  black  ash,  has  a  faint  odor  when  worked,  but 
for  all  practical  purposes  is  considered  odorless  and  tasteless. 

All  three  species  have  definite  annual  rings  made  very  conspicuous 
by  several  rows  of  large  pores  in  the  springwood.  In  the  summer- 
wood  the  pores  are  few,  very  small,  and  isolated,  or  occasionally 
two  or  three  in  a  radial  row.  In  the  white  and  green  ashes  the 
summerwood  pores  are  surrounded  by  light-colored  tissue  (paren- 
chyma) which  projects  tangentially,  producing  light-colored  lines 
often  joining  pores  somewhat  separated,  especially  in  the  outer  por- 
tion of  the  annual  ring.  (This  is  not  shown  as  clearly  in  the  illustra- 
tion of  white  ash,  fig.  29,  as  it  appears  on  a  smoothly  cut  end  surface.) 
In  black  ash  wood  the  parenchyma  is  scant,  and  projects  little,  if  any, 
from  the  pores.  (Compare  figs.  29  and  30.) 

The  rays  in  all  the  ashes  are  too  fine  to  be  distinctly  visible  with- 
out a  lens. 

Chestnut  resembles  the  heartwood  of  the  ashes,  especially  black 
ash,  but  is  lighter  in  weight  and  has  comparatively  many  pores  in 
the  summerwood,  the  pores  being  arranged  in  radial  groups.  (Com- 
pare figs.  24  and  30.)  Elm  can  be  distinguished  from  ash  by  the 
arrangement  of  the  numerous  summerwood  pores  in  wavy  tangential 
lines ; 

THE   HICKORIES. 

The  following  species  of  hickory  are  used  commercially: 

TRUE   HICKORIES. 

SHAGBARK  (Hicoria  ovata).    (B,  D,  E.) 
BIG  SHELLBARK  (Hicoria  laciniosa).    (B,  D.) 
PIGNUT  (Hicoria  glabra).    Black  hickory.    (B,  D,  E.) 
MOCKERNUT  (Hicoria  alba).    (B,  D,  E.) 

PECAN  HICKORIES. 

BITTERNUT  (Hicoria  minima).    Pignut  hickory.     (A,  B,  D,  E.) 
PECAN  (Hicoria  pecan).    (Western  parts  of  D  and  E.) 
NUTMEG  HICKORY  (Hicoria  myristicseformis).     (E.) 
WATER  HICKORY  (Hicoria  aquatica).     (E.) 


FIG.  28.— Hackberry.    Cross  section  magnified  15  diameters. 


FIG.  29.— White  ash.    Cross  section  magnified  15  diameters. 
84727—19 6 


FIG.  30.— Black  ash.    Cross  section  magnified  15  diameters. 


FIG.  31.— Shagbark  (hickory).    Cross  section  magnified  15  diameters. 


FIG.  32.— Black  walnut.    Cross  section  magnified  15  diameters. 


FIG.  33.— (True)  mahogany.    Cross  section  magnified  15  diameters. 


INFORMATION   FOR  INSPECTORS  OF   AIRPLANE   WOOD.  63 

The  wood  of  the  hickories  is  very  heavy,  hard,  and  tough  as  a  rule, 
except  water  hickory,  which  usually  is  lighter  in  weight  and  not  as 
strong.  The  sapwood  is  several  inches  wide.  The  heartwood  is 
reddish-brown  and  is  without  characteristic  odor  or  taste. 

The  annual  rings  are  clearly  defined  by  a  zone  of  larger  pores  in 
the  springwood,  except  in  water  hickory,  in  which  the  pores  decrease 
in  size  only  slightly  in  the  summer  wood  thus  making  it  diffuse- 
porous.  The  pores  in  the  summer  wood  are  few  and  isolated.  The 
most  characteristic  feature  of  the  hickories  is  the  numerous  fine, 
light-colored  tangential  lines  (parenchyma)  in  each  annual  ring. 
A  lens  is  necessary  to  see  these  lines  distinctly.  The  rays  are  not 
visible  without  a  lens.  (See  fig.  31.) 

Hickory  is  not  easily  confused  with  other  woods.  The  great 
hardness  and  fine  lines  of  parenchyma  distinguish  it  from  other 
commercial  woods. 

BLACK  WALNUT  (Juglans  nigra).     (B,  D,  E.) 

Black  walnut  is  heavy,  hard,  and  usually  straight-grained.  The 
heartwood  has  a  rich  chocolate-brown  color.  The  sapwood  is 
lighter  colored  but  tinged  with  some  of  the  color  of  the  heartwood. 

The  annual  rings  are  fairly  distinct,  for  the  pores  in  the  spring- 
wood  gradually  decrease  in  size  toward  the  outer  limit  of  each  annual 
ring.  Most  of  the  pores  are  visible  without  a  lens,  but  the  rays  are 
very  fine.  (See  fig.  32.) 

The  color  and  distinct  pores  are  usually  sufficient  to  distinguish 
black  walnut  from  all  other  woods. 

BUTTERNUT  (Juglans  dnered),  or  white  walnut.     (A,  B,  D.) 

Butternut  resembles  black  walnut  in  structure  but  is  lighter  in 
weight,  softer,  and  lighter  colored,  resembling  black  ash  or  chestnut 
in  color. 

(TRUE)  MAHOGANY  (Swietenia  mahogani).    (Native  in  southern  Florida,  southern 
Mexico,  Central  America,  northern  South  America,  and  the  West  Indies.) 

Mahogany  has  a  lustrous  reddish  brown  appearance  turning  darker 
on  exposure  to  the  air  for  a  long  time.  It  varies  greatly  in  weight. 
The  lighter  pieces  are  often  classed  separately  as  "baywood."  The 
wood  is  practically  odorless  and  tasteless,  which  distinguishes  it  from 
Spanish  cedar,  or  cigar-box  cedar  (Cedrela  odorata),  which  has  a 
characteristic  odor. 

The  growth-rings  (probably  annual  rings)  are  widely  variable  in 
width  from  one-thirty-second  to  one-half  inch  or  more.  They  are 
defined  by  fine  light-colored  lines.  (See  fig.  33.)  The  pores  are 
plainly  visible  without  a  lens  and  uniformly  distributed  throughout 
the  growth-ring,  appearing  as  grooves  on  the  longitudinal  surfaces. 
Numerous  pores  contain  dark  amber-colored  gum.  The  rays  are 
also  distinct  without  a  lens. 


64  INFORMATION  FOE  INSPECTORS   OF  AIRPLANE   WOOD. 

African  mahogany  resembles  true  mahogany  more  than  any  other 
species.  It  has  about  the  same  color  and  the  pores  are  fully  as  dis- 
tinct without  a  lens;  but  the  fine  light-colored  lines  which  define  the 
growth-rings  are  missing,  thus  affording  an  easy  method  of  distin- 
guishing the  two  species.  (Compare  figs.  33  and  34.) 

Red  gum  and  birch  can  be  stained  so  as  to  imitate  mahogany 
surprisingly  well,  but  on  close  inspection  it  will  be  found  that  the 
pores  of  red  gum  and  birch  are  not  distinctly  visible  without  a  lens. 
AFRICAN  MAHOGANY  (Khaya  sp.).  (West  coast  of  Africa.) 

African  mahogany  is  related  to  true  mahogany  in  the  same  way 
that  cigar-box  cedar  is ;  that  is,  it  belongs  to  the  same  family  but  to 
a  different  genus.  It  resembles  true  mahogany  in  color,  size  of  pores, 
medullary  rays,  and  workability,  but  differs  in  having  no  well-defined 
growth-rings.  (See  fig.  34.)  Occasionally  narrow  zones  of  less 
porous  wood  occur  but  these  must  not  be  confused  with  the  sharply 
defined  white  lines  found  in  true  mahogany.  The  weight  of  the  two 
species  averages  about  the  same. 

Like  true  mahogany  the  African  species  has  pores  plainly  visible 
without  a  lens,  many  of  the  pores  containing  a  dark  amber-colored 
gum. 
BEECH  (Fagus  atropunicea).    (A,  B,  D,  E,  and  eastern  half  of  C.) 

Beech  is  a  heavy,  hard  wood,  without  characteristic  odor  or  taste. 
The  heartwood  has  a  reddish  tinge  varying  from  light  to  moderately 
dark.  The  sapwood  is  usually  several  inches  wide  and  passes  grad- 
ually into  the  heartwood. 

The  pores  are  invisible  without  a  lens  and  decrease  in  size,  slightly 
and  gradually,  from  the  inner  to  the  outer  portion  of  each  ring. 
(See  fig.  35.) 

Some  of  the  rays  are  broad,  being  fully  twice  as  wide  as  the  largest 
pores  and  appearing  on  the  radial  surface  as  conspicuous  reddish 
brown  "flakes"  about  one-eighth  inch  wide  with  the  grain.  The 
other  rays  are  very  fine. 

Maple  resembles  beech,  except  that  in  maple  the  widest  rays  are 
about  the  same  width  as  the  largest  pores  and  not  so  conspicuous  on 
the  radial  surface.  Sycamore  (Platanus  occidentalis)  resembles  beech 
in  structure;  but  the  rays  in  sycamore  are  all  broad  and  therefore 
appear  more  numerous,  and  the  wood  is  considerably  lighter  in  weight. 

BLACK  CHERRY  (Prunus  serotina).    (A,  B,  C,  D,  E.) 

The  wood  of  the  black  cherry  is  moderately  heavy,  fairly  straight- 
grained,  and  without  characteristic  odor  or  taste.  The  sapwood  is 
narrow.  The  heartwood  has  a  lustrous  reddish-brown  color. 

The  annual  rings  are  fairly  well  defined  on  account  of  the  slightxy 
larger  pores  of  its  springwood,  which  decrease  in  size  gradually 


FIG.  34.— African  mahogany.    Cross  section  magnified  15  diameters. 


FIG.  35.— Beech.    Cross  section  magnified  15  diameters. 


FIG.  37.— Sugar  maple.    Cross  section  magnified  15  diameters. 


INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD.  65 

toward  the  outer  limit  of  each  annual  ring.  The  pores  are  not 
visible  without  a  lens. 

The  rays  are  very  distinct  on  the  end  surface,  the  larger  ones 
being  as  wide  as  the  largest  pores.  (See  fig.  36.) 

Cherry  is  easily  distinguished  from  most  other  woods  by  its  color. 
Mahogany  has  a  similar  color,  but  the  pores  in  mahogany  are  easily 
visible  without  a  lens. 

THE    MAPLES. 

Sugar  maple  (Acer  saccharum)  is  heavy,  hard,  and  difficult  to  cut 
across  the  grain,  in  which  respects  it  differs  from  the  soft  maples, 
silver  maple  (Acer  saccharinum—A,H,  D,  E),  and  red  maple  (Acer 
rubrum — A,  B,  D,  E,  and  eastern  half  of  C),  which  are  not  quite  as 
heavy  and  hard.  The  sapwood  is  wide  in  all  the  maples,  and  is  often 
sold  separately  as  " white  maple. "  The  heartwood  is  light  reddish 
brown,  without  characteristic  odor  or  taste. 

The  annual  rings  are  defined  by  a  thin  reddish  layer  usually  more 
conspicuous  on  dressed  longitudinal  surfaces. 

The  pores  are  all  very  small  and  uniformly  distributed  throughout 
the  annual  rings.  (See  fig.  37.) 

The  rays  are  very  distinct  without  a  lens,  and  under  a  lens  the 
largest  ones  appear  fully  as  wide  as  the  largest  pores.  On  radial 
surfaces  the  rays  are  conspicuous  as  small  reddish  brown  " flakes" 
about  one-thirty-second  to  one-sixteenth  inch  wide  with  the  grain. 
In  sugar  maple  only  part  of  the  rays  are  as  wide  as  the  pores;  the 
others  are  very  fine,  being  barely  visible  with  a  lens.  In  both  the 
soft  maples  all  the  rays  are  broad,  although  usually  not  quite  so 
broad  as  the  large  ones  of  sugar  maple.  However,  they  give  the 
appearance  of  being  more  numerous.  This  is  a  rather  fine  distinction 
and  an  inspector  should  have  samples  for  comparison. 

Birch  and  beech  resemble  maple  somewhat,  although  a  little 
experience  with  the  woods  will  readily  show  the  difference.  Birch 
has  larger  pores,  visible  as  fine  grooves  on  dressed  surfaces,  and  the 
rays  on  the  end  surface  are  not  distinctly  visible  without  a  lens. 
In  beech  some  of  the  rays  are  very  conspicuous,  being  fully  twice  as 
wide  as  the  largest  pores,  and  the  pores  decrease  in  size  and  number 
toward  the  outer  part  of  each  annual  ring. 

THE    BIRCHES. 

YELLOW  BIRCH  (Betula  lutea).    Gray  birch.'    (A,  B,  C.) 
SWEET  BIRCH  (Betula  lento).    Cherry  birch.     (A,  B,  D.) 

The  woods  of  the  yellow  birch  and  sweet  birch  are  so  much  alike 
that  as  a  rule  no  distinction  is  made  between  the  two.  The  yellow 
birch  is  the  more  abundant  of  the  two.  The  heartwood  of  both  is 


66  INFORMATION  FOE  INSPECTORS  OF  AIRPLANE  WOOD. 

marketed  as  "red  birch'"  and  the  sapwood  as  "yellow  birch."  The 
wood  is  heavy,  fairly  straight-grained,  and  without  characteristic 
odor  or  taste.  The  sapwood  is  wide  and  almost  white.  The  heart- 
wood  is  light  to  moderately  deep  reddish-brown. 

The  annual  rings  are  rather  indistinct.  The  pores  are  of  almost 
uniform  size  throughout  the  annual  rings  and  barely  visible  under  a 
good  light  and  on  a  very  smoothly  cut  end  surface.  (See  fig.  38.) 
On  dressed  longitudinal  surfaces  the  pores  appear  as  fine  grooves. 
The  rays  are  not  distinct  without  a  lens. 

Maple  is  occasionally  confused  with  birch;  but  the  two  are  easily 
distinguished  by  the  fact  that  in  maple  the  pores  are  much  smaller 
and  the  rays  wider,  being  very  distinct  without  a  lens. 

RED  GUM  (Liquidambar  styradflua).    Sweet  gum.     (B,  southern  part  of  D,  E.) 

Red  gum  is  moderately  heavy,  somewhat  lock-grained,  and  without 
characteristic  odor  or  taste.  The  sapwood  (sold  as  "sap  gum")  is 
highly  variable  in  width.  It  is  white  with  a  pinkish  hue  or  often 
blued  with  sap  stain.  The  heartwood  is  reddish  brown,  often  with 
irregular  darker  streaks.  The  wood  has  a  very  uniform  structure. 
The  annual  rings  and  pores  are  not  distinct  to  the  unaided  eye,  but 
the  rays  are  fairly  distinct  without  a  lens.  (See  fig.  39.) 

The  uniform  structure,  interlocked  grain,  and  reddish-brown  color 
are  usually  sufficient  to  distinguish  red  gum  from  other  woods. 
YELLOW  POPLAR  (Liriodendron  tulipijera).    WMtewood,  Tulip  poplar.    (B,  D,  E.) 

Yellow  poplar  is  moderately  light,  straight-grained,  and  without 
characteristic  odor  or  taste.  The  sapwood  is  from  1  inch  to  several 
inches  wide.  The  heartwood  is  light  to  moderately  dark  yellowish- 
brown  with  a  greenish  tinge. 

The  annual  rings  are  limited  by  light-colored  lines.  The  pores  are 
evenly  distributed  throughout  the  annual  ring,  and  are  too  small  to  be 
visible  with  the  unaided  eye.  The  rays  are  distinct  without  a  lens. 
(See  fig.  40.) 

Cucumber   (Magnolia  acuminata)  is  easily  confused  with  yellow 
poplar  and  is  usually  sold  as  such,  although  it  averages  slightly 
heavier  in  weight.     It  grows  in  the  same  region,  except  Florida  and 
the  South  Atlantic  coast. 
B  ASS  WOOD  (Tilia  americana).    Linden.     (A,  B,  C,  D,  E.) 

Basswood  is  a  light,  soft,  straight-grained  wood  with  a  creamy 
brown  color.  The  heartwood  is  not  clearly  defined  from  the  sap- 
wood.  It  is  without  taste,  but  Has  a  slight  characteristic  odor  even 
when  dry. 

The  wood  is  diffuse-porous,  the  pores  being  invisible  without  a 
lens.  The  rays  are  fairly  distinct  on  the  end  surface  and  often  con- 
spicuous on  the  radial  surface.  (See  fig.  41). 


f •/.;   • 


FIG.  38.— Yellow  birch.    Cross  section  magnified  15  diameters. 


FIG.  39.— Red  gum.    Cross  section  magnified  15  diameters. 


FIG.  40. — Yellow  poplar.    Cross  section  magnified  15  diameters. 


FIG.  41. — Basswood.    Cross  section  magnified  15  diameters. 


FIG.  42.— Cottonwood.    Cross  section  magnified  15  diameters. 


FIG.  43.— Western  red  cedar.    Cross  section  magnified  15  diameters. 


INFORMATION  FOB  INSPECTORS  OF  AIRPLANE   WOOD.  67 

Cottonwood  resembles  basswood,  but  is  more  grayish  in  color,  has 
larger  pores,  and  very  fin 3  rays  not  visible  without  a  lens.  Buckeye 
(jEsculus  octandra,  B,  D)  resembles  basswood  in  color  and  texture, 
except  that  the  rays  are  much  finer  and  are  visible  only  with  a  good 
lens. 

COTTONWOOD  (Populus  deltoides).    Poplar.     (B,  C,  D,  E,  F.) 

Cottonwood  is  light  and  fairly  straight-grained  (occasionally  some- 
what cross-grained).  The  straight-grained  lumber  warps  less,  and  is 
believed  to  come  from  old  slow-growing  trees  known  as  "yellow 
cottonwood"  in  distinction  from  the  "  white  cottonwood,"  which 
usually  has  wide  annual  rings  and  is  more  subject  to  warping.  Cot- 
tonwood is  without  taste  but  has  a  slight  characteristic  odor.  The 
heartwood  is  light  grayish-brown  not  clearly  defined  from  the  sap- 
wood. 

The  wood  is  diffuse-porous,  the  pores  being  very  numerous  and, 
under  favorable  conditions,  barely  visible  without  a  lens,  appearing 
as  fine  grooves  on  finished  lumber.  The  rays  are  very  fine,  barely 
visible  with  a  lens.  (See  fig.  42.) 

Cotton  gum,  or  tupelo  (Nyssa  aquatica,  E),  resembles  cotton- 
wood,  but  usually  is  heavier,  and  has  smaller  pores.  Yellow  poplar 
is  similar  in  weight  and  hardness,  but  its  greenish  tinge  usually  dis- 
tinguishes it.  Basswood  has  more  of  a  creamy  white  color,  smaller 
pores,  and  more  distinct  rays. 

The  swamp  cottonwood  (Populus  heterophylla)  of  the.  Southern 
States  and  the  black  cottonwood  (Populus  trichocarpa)  found  west  of 
the  Rocky  Mountain  region  have  the  same  appearance  and  properties 
as  the  common  cottonwood  (Populus  deltoides). 

CONIFERS. 

THE  CEDARS. 

Western  red  cedar  (Thuja  plicata,  H)  is  light  and  straight-grained. 
The  sapwood  is  rarely  over  1  inch  wide.  The  heartwood  is  reddish- 
brown,  with  the  characteristic  odor  of  cedar  shingles  and  a  somewhat 
bitter  taste  when  chewed.  The  wood  is  not  " pitchy"  and  contains 
no  resin  ducts,  although  it  contains  a  slight  quantity  of  aromatic  oils 

The  annual  rings  are  distinct,  moderate  in  width,  with  a  thin,  but 
well-defined  band  of  summerwood.  Pores  are  entirely  absent,  and 
the  rays  are  very  fine.  (See  fig.  43.) 

Arbor- Vit&  (Thuja  occidentalism,  or  northern  white  cedar  (A,  B,  C), 
resembles  the  western  red  cedar  in  odor  and  taste,  but  usually  it  is 
without  the  reddish  hue,  has  very  narrow  annual  rings,  and  averages 
lighter  in  weight. 


68  INFORMATION   FOE   INSPECTORS   OF   AIRPLANE   WOOD. 

Incense  Cedar  (Lilocedrus  decurrens)  (southern  half  of  H  and  I), 
is  also  very  similar  to  the  western  red  cedar,  although  it  has  wider 
sap  wood  as  a  rule.  Particles  of  amber-colored  resin  in  the  cells,  as 
seen  on  the  radial  surface  with  a  good  hand  lens,  are  abundant  in 
incense  cedar,  but  almost  absent  in  the  western  red  cedar.  Incense 
cedar  has  more  of  the  spicy  odor  and  taste,  and  the  wood  is  firmer 
than  that  of  the  western  red  cedar;  but  these  distinctions  are  relative 
and  can  be  applied  only  by  comparing  samples  of  the  two  species. 

The  wood  of  Port  Orford  Cedar  (CTiamsecyparis  lawsoniana)  (H),  also 
known  as  Lawson  cypress,  is  moderately  light,  straight-grained,  and 
with  a  pronounced  spicy  odor  and  taste.  The  sap  wood  is  not  clearly 
defined  from  the  heartwood  because  of  the  pale-brown  color. 

The  summerwood  is  not  dense  and  hard,  as  in  many  coniferous 
woods,  and  the  springwood  is  a  little  firmer  than  in  the  western  red 
cedar,  thus  making  Port  Orford  cedar  a  wood  very  uniform  in  struc- 
ture and  less  spongy  than  some  of  the  other  cedars.  (See  fig.  44.) 

The  odor  and  light-brown  color  are  usually  enough  to  identify 
Port  Orford  cedar. 

Yellow  Cedar  (Chamsecyparis  nootkatensis) ,  or  Alaska  cypress  (H), 
resembles  Port  Orford  cedar  in  weight,  structure,  and  odor,  but  is 
almost  clear  yellow  in  color. 

THE  WHITE  PINES. 

EASTERN  WHITE  PINE  (Pinus  strobus).    (A,  B,  C.) 

WESTERN  WHITE  PINE  (Pinus  monticola).    Idaho  white  pine.     (F,  H,  I.) 

The  above  two  species  of  the  white-pine  group  are  very  similar  in 
the  character  of  the  wood.  They  are  moderately  light,  straight- 
grained,  and  practically  tasteless,  but  have  a  slight,  yet  distinct, 
resinous  odor.  The  sapwood  varies  from  one  to  several  inches  in 
width.  The  heartwood  is  creamy-brown  to  light  reddish-brown, 
especially  reddish  at  knots. 

The  annual  rings  are  distinct,  but  the  summerwood  is  not  a  pro- 
nounced darker  or  appreciably  harder  layer.  Through  a  lens  the 
resin  ducts  appear  on  the  end  surface  as  specks,  or  minute  openings 
(see  fig.  45)  and  on  the  longitudinal  surfaces  often  as  yellowish- 
brown  lines.  Exudations  of  resin  are  common. 

Since  the  eastern  and  western  white  pines  are  very  similar  in 
appearance  and  properties,  it  is  not  necessary  to  distinguish  between 
the  two  commercially.  The  outer  portion  of  western  yellow-pine 
logs  has  narrow  annual  rings  with  a  very  thin  layer  of  summerwood 
(see  fig.  21),  approximating  the  white  pines  in  appearance,  and  is 
often  sold  as  "white  pine."  However,  it  can  be  distinguished  by 
its  horny,  glistening  layer  of  summer  wood,  especially  in  the  wider 
rings  in  which  it  is  more  conspicuous  (compare  figs.  45  and  47). 


FIG.  44.— Port  Orford  cedar     Cross  section  magnified  15  diameters 


FIG.  45.— Western  white  pine.    Cross  section  magnified  1  s  diameters. 
84727—19 7 


FK, 


46. — Sugar  pine.    Cross  section  magnified  15  diameters. 


FIG    7  —Western  yellow  pine.    Cross  section  magnified  15  diameters. 


INFORMATION  FOR  INSPECTORS  OF  AIRPLANE  WOOD.  69 

Often  the  summerwood  is  more  distinct  on  the  longitudinal  surface; 
and  in  a  shipment  of  western  yellow-pine  lumber  numerous  boards 
with  conspicuous  layers  of  summer  wood  may  be  found. 
SUGAR  PINE  (Pinus  lambertiana).     (I.) 

Sugar  pine  is  very  much  like  the  white  pines  in  structure  and 
properties,  and  in  fact  belongs  to  the  white-pine  group  botanically. 
The  sapwood  is  from  one  to  several  inches  wide.  The  heartwood  is 
very  light  brown,  only  slightly  darker  than  the  sapwood  and  prac- 
tically never  reddish,  as  is  the  case  quite  often  in  the  white  pines. 
Brown  stain  is  common,  and  is  caused  by  drying  the  lumber  under 
certain  conditions.  The  summerwood  never  appears  as  a  horny, 
glistening  band  as  in  the  hard  pines. 

The  wood  of  sugar  pine  has  a  slightly  coarser  texture  than  that  of 
white  pine;  that  is,  the  fibrous  cells  and  also  the  resin  ducts  have  a 
greater  average  diameter.  The  distinction  is  rather  fine,  however, 
to  use  without  a  compound  microscope.  (See  fig.  46.)  On  a  longi- 
tudinal surface  the  resin  ducts  are  usually  more  conspicuous  as 
brownish  lines,  but  in  some  pieces  no  more  so  than  in  the  white  pines. 

Resinous  exudations,  which  become  granular  and  have  a  sweetish 
taste,  are  quite  common  in  sugar-pine  lumber,  and  when  present  are 
the  most  reliable  means  of  distinguishing  it  from  other  pines. 

THE    YELLOW   OR   HARD    PINE    GROUP. 

WESTERN  YELLOW  PINE  (Pinus  ponderosa).     "California  white  pine."     (F,  G,  H,  I.) 
NORWAY  PINE  (Pinus  resinosa).     Red  pine.     (A,  C,  and  northern  half  of  B.) 
SHORTLEAF  PINE  (Pinus  echinata}.     (E.) 
LOBLOLLY  PINE  (Pinus  treda).     (E.) 
LONGLEAF  PINE  (Pinus  palustris).    (E.) 

The  yellow  pines  are  mostly  heavier,  harder,  more  resinous,  and 
contain  a  wider  and  harder  layer  of  summerwood  than  the  white 
pines.  However,  exceptions  occur,  notably  western  yellow  pine, 
which  in  the  outer  part  of  the  mature  trees  is  often  as  light  in  weight 
as  the  average  white  pine.  The  sapwood  is  variable  in  width  in  the 
different  species  and  even  in  the  same  species,  averaging  narrowest 
in  longleaf  pine.  The  heartwood  is  orange-brown  to  reddish-brown 
in  color. 

The  summerwood  is  usually  defined  as  a  conspicuously  denser, 
harder,  and  darker  band  of  rings  of  average  width  (see  Shortleaf 
pine  in  PI.  II,  opposite  p.  50);  but  in  very  narrow  rings,  such  as 
are  found  in  the  sapwood  of  old  trees  of  western  yellow  pine,  the 
summerwood  layer  is  very  narrow  and  inconspicuous.  (See  figs.  21 
and  47.) 

The  resin  ducts  are  plainly  visible  with  a  lens  on  a  smoothly  cut 
end  surface,  and  often  appear  as  brownish  lines  on  the  longitudinal 
surfaces.  Exudations  of  resin  are  common  in  the  yellow  pines. 


70  INFORMATION   FOR  INSPECTORS   OF   AIRPLANE   WOOD. 

No  reliable  means  of  distinguishing  all  the  yellow  pines  without  a 
compound  miscroscope  is  known.  The  order  in  which  the  above 
species  are  listed  indicates  on  the  average  their  relative  weight,  in- 
creasing from  western  yellow  pine  to  longleaf  pine.  Exceptions  occur, 
however;  for  instance,  some  shortleaf  pine  is  heavier  than  the  average 
longleaf. 

Douglas  fir  is  somewhat  similar  to  yellow  pine  in  appearance,  but 
usually  has  a  distinct  reddish  hue  as  contrasted  with  the  orange- 
brown  color  of  yellow  pine  (see  also  Douglas  fir). 
DOUGLAS  FIR  (Pseudotsuga  taxifolia).    Oregon  pine.     Douglas  spruce.     (F,  G,  H,  I.) 

Douglas  fir  differs  from  other  firs  (white  fir,  noble  fir,  grand  fir, 
balsam  fir,  etc.)  in  being  resinous,  heavier,  stronger,  and  in  having  a 
distinctly  darker  heartwood.  The  heartwood  has  a  reddish  hue, 
usually  quite  pronounced  although  in  old  coast  firs  the  outer  part  of 
the  heartwood  is  less  reddish  and  is  marketed  as  "  yellow  fir."  The 
sapwood  is  from  one  to  several  inches  wide. 

The  annual  rings  are  made  distinct  by  a  conspicuous  band  of  sum- 
merwood.  Resin  ducts  are  present  but  not  so  distinct  as  in  the  pines, 
usually  appearing  as  whitish  specks  in  the  summerwood.  Often 
several  ducts  are  in  short,  tangential  rows,  a  feature  never  found  in 
the  pines. ,  (Compare  figs.  47  and  48.)  Exudations  of  resin  are  com- 
mon or  can  usually  be  made  to  appear  slightly  by  warming  the  wood. 

The  wood  of  western  larch  (Larix  occidentals)  (F,  H)  resembles 
that  of  Douglas  fir  considerably,  but  has  narrower  sapwood  (rarely 
over  1  inch)  and  lacks  the  reddish  color  of  the  fir.  The  resin  ducts 
in  the  two  species  are  of  about  the  same  character,  although  the  fir 
is  more  resinous  as  a  rule.  With  a  compound  miscroscope  Douglas 
fir  can  easily  be  distinguished  from  all  other  commercial  woods  by 
the  presence  of  fine  spirals  in  the  cells,  similar  to  the  thread  in  a  nut. 
This  can  be  seen  on  a  longitudinally  split  surface  without  preparing 
a  miscroscopic  slide. 

SPRUCE. 

WHITE  SPRUCE  (Picea  canadensis).     (A,  B,  C.) 

RED  SPRUCE  (Picea  rubens).    (A,  B.) 

SITK A  SPRUCE  (Picea  sitchensis}.    Tideland  spruce.     (H.) 

The  spruces  are  moderately  light,  straight-grained  woods.  In  'the 
white  spruce  and  red  spruce  the  heartwood  is  as  light  colored  as  the 
sapwood,  but  in  Sitka  spruce  the  heartwood  has  a  light  reddish  tinge, 
making  it  a  little  darker  than  the  sapwood. 

The  annual  rings  are  clearly  defined  by  a  distinct,  but  not  horny, 
band  of  summerwood.  Spruce  resembles  the  white  pines  in  texture , 
but  the  resin  ducts  are  fewer  and  smaller  in  spruce  (compare  figs.  44 
and  48),  usually  appearing  as  whitish  specks  in  the  summerwood. 


FIG.  48.— Douglas  fir.    Cross  section  magnified  15  diameters. 


FIG.  49.— Sitka  spruce.    Cross  section  magnified  15  diameters. 


FIG.  50.— Bald  cypress.    Cross  section  magnified  15  diameters. 


FIG.  51.— Redwood.    Cross  section  magnified  15  diameters. 


SI TK A    SPRUCE 


13 


FIG.  52.— Split  tangential  surfaces  of  Sitka  spruce  and  Douglas  fir  showing  the  "pocked"  or  dimpled  appearance  of  the 
spruce  not  found  in  Douglas  fir.  This  characteristic  is  most  pronounced  in  Sitka  spruce  with  narrow  rings  and  is 
almost  entirely  absent  in  very  wide-ringed  material. 


INFORMATION  FOE  INSPECTORS  OF  AIRPLANE  WOOD.  71 

Pitch  pockets  are  occasionally  found  in  spruce,  and  slight  exudations 
of  resin  occur  on  cuts  made  before  seasoning  the  wood. 

On  account  of  its  reddish  tinge,  Sitka  spruce  might  be  confused 
with  light  grades  of  Douglas  fir.  However,  the  fir  has  denser  sum- 
merwood  except  in  very  narrow  rings;  therefore,  rings  of  average 
width  should  be  compared.  Tangentially  split  surfaces  of  Sitka 
spruce  usually  have  numerous  slight  indentations  which  give  it  a 
"pocked"  or  dimpled  appearance  never  found  in  Douglas  fir  (see  fig. 
50).  This  characteristic  is  more  pronounced  in  material  with  narrow 
annual  rings,  and  may  be  missing  entirely  in  wide-ringed  spruce, 
especially  that  near  the  center  of  the  tree.  *  Occasionally,  the  eastern 
spruces  also  show  this  uneven  tangential  surface  to  a  slight  extent. 
Some  trees  of  western  yellow  pine  also  develop  this  "pocked" 
appearance  of  the  wood,  but  the  pine  can  be  distinguished  by  the 
larger  and  more  numerous  resin  ducts. 

BALD  CYPRESS  (Taxodiun  distichum).    (E.) 

Cypress  is  highly  variable  in  color  and  weight.  Commercially,  the 
wood  is  often  classified  as,  "white,"  "yellow,"  "red,"  or  "black" 
cypress,  although  it  is  all  derived  from  the  same  botanical  species. 
As  a  rule,  the  darker  grades  are  heavier,  but  this  is  not  always  the 
case.  The  wood  has  a  characteristic  rancid  odor  when  fresh.  In 
dry  wood  the  odor  is  less  pronounced,  but  can  be  detected  by  whit- 
tling it  or,  better  yet,  sawing  it  and  holding  the  sawdust  to  the  nos- 
trils. The  wood  is  without  characteristic  taste. 

The  annual  rings  usually  are  irregular  in  width  and  outline.  The 
summerwood  is  very  distinct  but  narrow,  although  wider  than  in  the 
cedars  (see  fig.  50).  Cypress  feels  greasy  or  waxy  to  the  touch, 
especially  the  heavier  and  darker  grades.  Resin  ducts  and  exuda- 
tions of  resin  are  absent. 

Cypress  resembles  the  cedars  and  redwood  somewhat;  but  the 
cedars  have  an  aromatic  odor  and  spicy  taste,  and  redwood  is  taste- 
less and  odorless. 

REDWOOD  (Sequoia  semperuirens) .    (I,  along  coast.) 

Redwood  is  moderately  light,  straight-grained,  and  obtainable  hi 
large  clear  pieces.  The  heartwood  is  deep  reddish-brown  in  color  as 
a  rule.  Occasionally,  lighter  colored  pieces,  resembling  western  red 
cedar,  are  found. 

The  wood  contains  no  resin  ducts.  The  annual  rings  are  made 
very  distinct  by  a  narrow  but  dense  band  of  summerwood  alternating 
with  soft,  spongy  springwood  (see  fig.  51).  Redwood  is  without 
characteristic  odor  or  taste.  The  lack  of  these  distinguishes  it  from 
the  cedars,  which  it  resembles  in  appearance  and  properties. 


72  INFORMATION   FOE  INSPECTORS  OF   AIRPLANE   WOOD. 

Publications  on  the  Nomenclature, Occurrence,  and  Structure  of  American 

Woods. 

1.  Government  publications. 

Check  List  of  Forest  Trees  of  the  United  States.    Forest  Service  Bulletin  17.    1898. 

15  cents. 
Timber:  An  Elementary  Discussion  of  the  Characteristics  and  Properties  of  Wood. 

Forest  Service  Bulletin  10.    1895.     10  cents. 
Guide  Book  for  the  Identification  of  Woods  Used  for  Ties  and  Timbers.    Special 

Forest  Service  Publication.    1917.    30  cents. 

NOTE.— The  above  publications  may  be  obtained  at  the  prices  indicated  from  the  Superintendent  of 
Documents.  Government  Printing  Office,  Washington,  D.  C. 

2.  Papers  prepared  by  the  Forest  Products  Laboratory  and  published  in  various 
journals: 

A  -Visual  Method  of  Distinguishing  Longleaf  Pine  (for  pieces  containing  the  pith),  by 
Arthur  Koehler. 

American  Lumberman,  September  11,  1915. 
Engineering  Record,  September  11,  1915. 

Py loses:  Their  Occurrence  and  Practical  Significance  in  Some  American  Woods,  by 
Eloise  Gerry. 

Journal  of  Agricultural  Research,  May  25,  1914. 

A  Plea  for  Closer  Discrimination  in  the  Use  of  the  Words  "Grain"  and   "Texture'' 
with  respect  to  Wood,  by  Arthur  Koehler. 
Hardwood  Record,  February  25,  1917. 


OVERDUE- 

___— -=====^ 


UNIVERSITY  OF  CAUFORNIA  LIBRARY 


I 

3 


